Trying to love two women is like a ball and chain. Same goes for two men, or any other possible combination.
Two-timing, playing away from home, having a bit on the side, going behind your partner's back, adultery, infidelity... There are a lot of names for cheating on your partner, but most of them have the same outcome: a world of hurt.
Most of us recognize this type of plot: Bob is married to Alice. One day, Bob sees Dorothy at a club and is attracted to her. Perhaps things haven't been going so well with Alice for some time. Maybe they just had a major fight and Bob stormed off. Or maybe his marriage is perfectly healthy, and Bob has no other excuse than his own selfishness/egotism/libido. Whatever the reason, Bob flirts with Dorothy, which eventually leads to a romantic relationship and the various things that entails. But here's the thing: Bob doesn't tell Alice about it. He doesn't dump her, he doesn't tell her that he thinks the marriage is on the rocks, he doesn't even ask for "more space". He continues to play the part of her husband, and expects her to continue being his wife, hoping that Alice won't notice when he starts coming in late for dinner, or ask him about the mysterious expenditures on their joint account. Sometimes, just to really play Alice for a sucker, their marriage will seemingly improve— he buys Alice gifts, pays attention to her and seems much happier, but all the while he's running off to see Dorothy. For extra scumbag points, he may be keeping Dorothy similarly in the dark about Alice. Chances are he'll eventually get caught; if he didn't, the story wouldn't have the same dramatic impact. A lot of angst and tension will ensue.
Way back in the day, when marriage was considered permanent and divorce was a word whispered fearfully by gossiping old ladies, The Affair was a shocker of a storyline, and very often an automatic Moral Event Horizon for the cheating partner. However, it's worth noting that even further back in the day, the gods, goddesses and minor side characters of mythology listed "infidelity" under "Hobbies", didn't particularly care if their new "partner" was willing, and got away with it. Only their mortal lovers got the nasty side of the wronged wife's/husband's temper when the affair was discovered.
Nowadays, affairs are common in any Soap Opera and turn up an awful lot in other types of story as well. We don't really expect a fictional husband and wife to stay faithful to each other for forty or so years. Supposedly, a solid marriage makes a boring story (though some would disagree). Often, a sequence of "get together -> one cheats -> they break up -> they make up -> the other one cheats", and so on) will be followed so often and so tiresomely that it becomes a Yoyo Plot Point.
What defines cheating usually depends on the context of the story and the characters involved. Stories aimed at younger audiences, or with a clear emphasis on romantic, monogamous relationships, will probably count kissing and flirting as cheating. Most examples of cheating in shows aimed at adults, however, will involve sex. Occasionally, characters may clash specifically because they have different definitions of cheating: for example, the husband who protests that he was drunk and it was "just a kiss" to his furious wife, or a girlfriend who can't understand why her boyfriend doesn't like her flirting with her male friends.
A few rules usually hold true in fiction: Women who cheat are generally portrayed much more sympathetically than men. The (male) big boss of any given workplace is likely to be two-timing his wife. The Protagonistremains sympathetic if they cheat, and becomes an innocent, wronged victim if they are the one being cheated on. Bisexuals are portrayed as incapable of faithfulness or have merely informed sexuality), and men are more prone to having affairs than women. note So those that do must be very busy.
Unfortunately, adultery is Truth in Television, as many broken hearts and broken families will testify.
See also Mistaken for Cheating, The Unfair Sex, The Mistress, Good Adultery, Bad Adultery. This is generally the #7 scenario of Triang Relations.
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Anime and Manga
In Victorian Romance Emma, William is technically guilty of this as well, despite being sympathetic. Told to give up on his love for Emma, he proposes (under duress, it has to be said) to a more "suitable" Proper Lady, Eleanor. On reuniting with Emma however, he continues their chaste love affair while he is still officially betrothed to Eleanor, until he finally breaks the engagement.
Cheating, or rather, paranoia about being cheated on, is a source of humour in Kyou Kara Maou — Yuri's jealous male fiance, Wolfram, calls him "cheater" so often it's practically a nickname. In fact, Yuri only has to be nice to anyone, of either sex, to set Wolfram on a rampage. Whether Yuri is actually capable of cheating, however, depends on whether the viewer considers their accidental engagement valid or not.
One example comes in the Fruits Basket manga. A teenaged Akito Sohma sleeps with her cousin Kureno in an attempt to ensure that he won't abandon her, having been released from the Zodiac curse - which made the already VERY unstable Akito fall into despair. When her other cousin Shigure finds out, he sleeps with her motherRen, partly to get back at Akito (who hates her extremely abusive mother) and partly for the family resemblance between mother and daughter. When Akito finds out, she flies into a jealous rage at Shigure and kicks him out of the Sohma main house, not forgiving him for several years.
Blue Eyes.. The main character Tatsuya, gets into the pants of 7 girls by the end of volume 9 (the last one made recently) and it's implied he might get even MORE. And his main girlfriend is only aware of one of these girls. The other five? She doesn't suspect a thing and neither do they.
Happens several times in Pet Shop of Horrors and its sequel series. Generally, this is enough to get one ripped up or horribly poisoned or killed by something. One noteworthy example was when the sweet-looking wife of a gang boss hired a hot woman for some purpose for her husband. The implication was that the woman was an assassin and would kill a rival mafia leader. By the end, it turned out that the hot woman was actually the human form of a Zhen, a bird who can turn any drink to poison, just by adding a feather. The wife was furious at her husband for constantly cheating on her and actually got the Zhen to kill him.
School Days. Makoto sleeps with pretty much every female character except for his initial girlfriend. He cheats on said girlfriend with another girl, and cheats on her with two more girls. It comes back to bite him. Fatally.
Soul: How should I know? Cool men don't cheat.* He says this showing he's now in a perfect position to turn into a scythe so Maka has a clear shot at Blair with him.
She got better after she got used to Blair living with them. Then, when the cast are subjected to a Gender Bender (it's a long story), boy!Maka got a nosebleed at the sight of the succubus, leading girl!Soul to a moment of All Men Are Perverts.
In Oniisama e..., Mariko Shinobu's father Hikawa, who is an erotic novel writer, not only had an affair with a former actress during his marriage with Hisako, but later left his wife for said former actress. This was the cause of Mariko's distrust towards men.
Discussed but ultimately averted with Prof. Misonoo. He turned to drinking when his first wife left him and took their kid Takehiko (the aforementioned Oniisama) away with her, and it was during this time that he met Nanako's mom, who was a beautiful waitress in his favorite pub and he fell in love with her. The letter proclaiming that Nanako to be kicked out from the Sorority accused Nanako's mom to be a homewrecker, but it turns out that it wasn't to be the case.
Played for Laughs in Sailor Moon, where Minako was parallelly dating two men: the sensitive artist Takano and the Badass Biker Torashima. Subverted when it turns out both dudes knew... and they were Hawk Eye and Tiger Eye, who wanted to take peeks at Minako's Dream Mirror, which was the reason why they both were after her. Hilariously, when Minako finds out about this... she says "How dare you guys lie to me?!" and they reply at unison "YOU HAVE NO RIGHT TO SAY THAT!".
Greg and later Ian from A Cruel God Reigns. Greg cheats on Sandra by raping her son, Jeremy and Ian when he starts to fall for Jeremy and pays him to have sex, even though he is still with Nadia.
Makie from Futari Ecchi constantly tries to get Makoto to have an affair with her.
In Deconstructing Harry, this has been the protagonist's M.O. for all his life, to the point he's broken up several marriages; the very start shows him cheating on his wife with her (married) sister. He later dumps both for a young, attractive admirer.
In Fatal Attraction, the cheating husband is played as a victim when his mistress turns out to be a scary stalker who puts his daughter's pet rabbit in a pressure cooker. In one ending she gets nailed with karmic death when her lover's wife shoots her, while we don't see what the wronged wife does to her cheating husband after the events of the movie. Given that the affair endangered her child (the mistress could very easily have gotten her hands on said child, and was crazy enough to pose a potential threat), the cross between Mama Bear and Woman Scorned would be the really scary part.
Mr. Destiny screws with this, when the main character (not the titular character) ends up in an alternate reality where he and his wife are married to different people.
Mary Jane Watson in the Spider-Man movies is usually cheating on someone. In the first film she cheats on Harry by kissing Spider-Man in the rain. In the second film she is engaged and cheats with Peter Parker. In the third film, she is indignant that Gwen kisses Peter upside down (though it was nothing more than a act for the audience) because that's "their kiss," even though when she first did it, she was the one cheating.
In Love Actually, Alan Rickman's character appeared to be emotionally cheating on his wife with his secretary by buying her an expensive necklace while buying his wife a CD. However, the CD was what his wife wanted (proving that he did love his wife) and at the end of the movie, despite his wife finding out about the necklace and him admitting to getting it for his secretary, they're not together but have reconciled.
One of the major plotlines in Swing Shift is Kay's eventual affair with Lucky while her husband is away at war. Naturally it turns out badly, though Kay and her husband do reconcile later.
In Eve's Bayou, one of the main plot points is 9 year old Eve catching her father with another married woman. Turns out her father often has affairs with his patients.
It's worth noting that Sam is actively resisting the other girl, who's coming on very strong. Then again, she's a cyborg sent by Megatron to extract Allspark data from Sam's mind.
Sam is also indignant after Mikaela walks in on the girl on top of Sam (metal can be very heavy to push off) and is angry at him for cheating. Sam is angry that Mikaela thinks him capable of cheating and considers himself the wronged party.
Happens all over the place in Forgetting Sarah Marshall. First we have the titular character cheating on her boyfriend Peter with the British Rock Star Aldous Snow (later, it's revealed that they've been sleeping together for a year before she told Peter). At dinner, Aldous starts talking and claims that he has a right to bed any woman he wants, which implies that he may have already done so while being with Sarah. Finally, after Aldous breaks up with Sarah, she tries to get back together with Peter, and they even start having sex, before Peter snaps back and leaves in mid-blowjob. Unfortunately for him, he then tries to explain himself to his new girlfriend Rachel, who promptly breaks up with him. Luckily, they get back together after a while.
Loverboy is about a college slacker (played by Patrick Dempsey) who, unexpectedly, finds himself in the escort business, sleeping with women (all older than him) whose husbands are ignoring them. Why? So he can pay for college and get back to his girlfriend. He sees nothing wrong with this up until the end, when she confronts him about it and is uncertain she wants to stay with him. Interestingly, she doesn't immediately shut him out and appears to understand that he meant well. Then there are his three primary clients, who are obviously cheating. Their husbands are implied (and one outright admits it) to also be cheaters. At the end, two of the couples get back together, seeming to forgive each other's "mistakes". One, though, decides that she's had quite enough of her husband who treats her "like a geisha" and leaves him to rot in jail.
In High Society, details of Seth Lord's infidelity are used as leverage by Spy Magazine to get exclusive coverage of his daughter's wedding, kicktstarting the film's plot.
While on an assigment Morgan Sullivan in Cypher tries to hit on a woman he met in the hotel he's staying at. After striking up a conversation with her, she calls him off when she notices his wedding ring, which he forgot to take off. Subverted. After discovering his real identity as Sebastian Rooks, it turns out that the women, Rita Foster, is his real lover, and his supposed "wife" was just one of his employees who he hired to play the part.
In Heat, Hanna catches his wife cheating on him as a result of his own obsessive devotion to pursuing Neilnote Not romantically, mind you. Hanna is simply a cop and Neil the criminal he's chasing., at the expense of his marriage; he takes it badly. Unexpectedly the scene has comedic elements. Charlene also cheats on Chris with Alan Marciano, so Hanna and Chris have something in common, though the difference is that it is Neil who discovers Charlene's secret while setting up the drop for the bonds with Van Zant.
This drives half the plot of True Lies. Harry discovers that his wife has been having an affair behind his back, but it's a subversion. The guy she was "cheating" with is a loser who was trying to get Helen to sleep with him, but she never does. Nonetheless Harry uses his spy agency's resources to bust her in the middle of meeting with the man he thinks she was cheating with, and recruits her (without disclosing his own identity) so she can have some excitement in her life.
In Red Riding Hood, it was discovered that Suzette's affair with Henry's father was the entire reason why the Wolf began its murderous rampage.
In The Mood For Love is about two neighbors who realize that their respective spouses are having an affair with one another, and become tentatively attracted to each other as well. And, true to form, there's a boss in the story with a mistress as well as a wife.
The Chairman from Memoirs of a Geisha is already married when he gets together with Sayuri, but in this case a man being married and having a danna relationship with a geisha was not frowned upon and was indeed a sign of status during that period. It wasn't cheating as we think of it.
In Stardust, Victoria despite promising to marry Tristan when he returned with a star got engaged to another a man while he was away. Of course, when she set the Engagement Challenge she hadn't expected him to actually go into Faerie and bring back the star, and did offer to break off her engagement and keep up her end of the bargain.
The unnamed father in Chapter 36 of The Pale King has multiple affairs. He gets off on the idea of women needing him.
Sisterhood series by Fern Michaels: A number of villains have engaged in this trope. Senator Webster in Payback. Rosemary Hershey in Sweet Revenge, who is female, and her cheating is in no way presented in a sympathetic light. Roland Sullivan in Lethal Justice. Mitch Riley in Hide And Seek. Baron Bell in Deadly Deals. A couple of them receive a divorce as one of the consequences. Interestingly, the trope gets deconstructed in Payback. It's like this...Senator Webster cheated on his wife Julia Webster with multiple women. She had him write down a list of the women he had affairs with. One of them is married, and Julia points out that her husband would have performed mayhem on the Senator if he had known. The real clincher is that the Senator got infected with AIDS from one of the women, and he ended up giving it to Julia! He didn't know he was infected, but still.... Of course, she did reveal the truth to him that they both have AIDS, and he naturally could not believe that powerful he had been infected by such a thing. It goes to show that recklessly sleeping around is not bliss, and that it is in fact dangerous!
It's hard to find an Agatha Christie novel that doesn't contain cheating or pretense thereof.
In Death on the Nile, Simon continues having an affair with Jaqueline while married to Linnet.
A Brother's Price: Keifer Porter. Combined with Lysistrata Gambit. With possibly many people, but definitely including hissister. There is such a thorough aversion of STD Immunity in this book that cheating - and possibly catching something and spreading it back to the group marriage, potentially letting the entire family fall ill and die - is a major concern.
It's par for the course for men to cheat on their wives in A Song of Ice and Fire - that's why there are so many bastards everywhere. The most notable situation is the one surrounding Cersei Lannister. Neither she nor her husband were even remotely faithful in their loveless Arranged Marriage, as she's sleeping with her brother and he's sleeping with everyone. When Jaime is imprisoned, Cersei finds solace with their cousin and some bannermen. Later, a close friend. Jaime is crushed when he finds out, especially since, as he points out, he's one of the only men in Westeros who's remained entirely faithful to the woman he loves (if not to the vow of celibacy he took as a member of the Kingsguard).
Ludovico has an uncomfortable moment where he realizes that there are four people in the room including himself... and they've all slept with his wife. On the other hand, when he also sleeps with other people to reach Palimpsest, it doesn't cross his mind that he's just as bad because he's doing it for the purpose of finding her.
When It Happens To You is primarily about the main couple's marriage falling apart due to the husbands infidelity.
Daniar and Jago have a history and Kalak thinks that history is in the present. Daniar did have a relationship and at one point carried his child but there is nothing between them in the story.
Kalak visits Zarracka's cell regularly and she tempts him with conjugal sex but he never accepts.
Redeeming Love: Oh boy. Angel’s story effectively hinges on this trope—her father allowed her to be sold into prostitution when she was eight because his wife had cheated on him and Angel wasn’t his child. Later on, Angel (temporarily) leaves her own husband to return to life as a prostitute (almost causing him to break his resolve never to hurt her), and she also sleeps with his brother shortly beforehand, although that was because he blackmailed her into it.
Your Cheating Heart: Played with. The Laws of Entropy send his wife’s former sexy classmate to visit the protagonist Malyanov, who is certainly tempted to cheat, but doesn’t give in. Later, the same laws make the classmate lose her bra at his flat to be found by his wife, which nearly causes a divorce and forces Malyanov to reveal the truth about the proceedings.
Jane Austen's Mansfield Park: Maria Bertram married Mr Rushworth even though she didn't love him. She hoped for a relationship with Henry Crawford even before her wedding, and when he flirted with her later, she couldn't resist. Their relationship went probably further than Henry anticipated. They are discovered by servants and Mr Rushworth's mother makes the affair public. Mr Rushworth is granted a divorce, and Henry and Maria live together for some time. However, he never intends to marry her, which crushes her and they break up. She's punished by having to live in disgrace and she cannot even hope that she could be a part of good, respectable society ever again.
Count Oblonsky's cheating on his wife Dolly kicks off the plot. Dolly is devastated because she never even considered it as a possibility and admits her naivety. They manage to reconcile, but Dolly is never truly happy again. Their children feel something is amiss as well.
Anna Karenina falls for Alexei Vronsky, and he falls for her. Hard. Anna struggles a bit, but she cannot resist her passion. Anna's husband becomes severely depressed when he learns of Anna's infidelity and finds it very difficult deciding whether he will officially divorce Anna, a socially risky move for him, her, and their son.
Most of the aristocrats and royalty, with the exception of a "pathetic" few, have had (numerous) affairs.
Madame Bovary: Emma's husband is a kind-hearted, yet naive idiot, and he doesn't even come close to Emma's imagination and intelligence. She has two lovers who are pleased about having such a pretty mistress but don't actually care for her that much as she cares for them. Her husband never learns of it while she's alive even though she's hardly discreet.
In Southern Sisters Mysteries, Alan cheats on Lisa offscreen at one point, resulting in Lisa briefly staying with Patricia Anne and Fred, which is how she becomes involved in the book.
Rory from Gilmore Girls first cheats on Dean with Jess (only a kiss involved, but they're all high-schoolers at the time and kissing is the hottest action anyone gets, even among couples), then consciously with Dean on his wife, Lindsay (sex involved) and then on Logan with Jess again (only kiss). She is called out on the second one (by her mother), but generally regardless of those actions she's portrayed as the series' "good girl".
Par for the course in Scrubs: Jordan's promiscuity is legendary before she finally settles down, Dr. Kelso has been having at least one affair at a time throughout his marriage, and J.D. and Turk are both tempted to cheat at various points in the story. At one point J.D. made the questionable decision to tell his girlfriend about how he could have cheated on her with the woman he'd lied to her about what he was doing that night to meet, but hadn't.
Pick any crime show or detective drama. About half to two-thirds of the time, the murder had to do with someone having an affair. (The rest of the time, it's usually about money.)
Used regularly on the comedyLast of the Summer Wine, principally (but not exclusively) with Howard and Marina. Here, the justification for using marital infidelity as comedy is that the characters are generally unattractive and lack the courage to take their affair to a very intimate level. And the fact that for all the absurd lengths they go to in order to cover it up, the wife knows full well what's going on and is not impressed.
Too many to count in Skins. The show has been criticised in some quarters for relying on cheating as the primary generator of conflict.
Series 1. Tony cheated on Michelle with many women and one man.
Sid cheated on Cassie with Michelle.
Michelle on Tony with Sid. Debatable as they split up in series 1.
Chris on Jal with Angie.
Pandora on Thomas with Cook.
Freddie on Katie with Effy.
Emily on Naomi with JJ. (Dubious, because they weren't entirely sure if they were actually dating or not, but Naomi certainly plays it that way.) Which turns Naomi into a hypocrite, since her and Emily's relationship at that point consisted of Naomi coming up with excuses that made it possible for her to consider herself straight in-between make-out sessions and her freaking out and leaving Emily in the middle of the night after their first time together.
Thomas on Pandora with Andrea.
Naomi on Emily with Sophia.
Emily on Naomi with that random at the barbeque.
Emily on Naomi with Mandy (although it took a while for Naomi to get it).
In Noah's Arc, Eddie cheats on Chance, Noah cheats on Wade, and Wade cheats on Dre.
Gwen has an affair with Owen in Torchwood. She feels really guilty about it, though, and tells Rhys... after spiking his drink with Retcon. While she wants to hear him forgive her, she doesn't really want this to end their relationship. She ends up breaking off the affair and marrying Rhys.
Much of the drama in Flash Forward is derived from Mark's wife Olivia having a vision of herself with another man in six months' time. Eventually, it destroys her marriage.
A standard plot-line in Gossip Girl, with literally too many instances to list and many of them too complicated to explain.
Prime example is Dan Humphrey. He's cheated on three out of four girlfriends.
Taub of House is a big offender, despite his recent efforts to stay faithful to his wife.
The New Normal: Goldie catches her husband sleeping with an Asian woman.
The early seasons of M*A*S*H take a pretty casual attitude towards adultery. Henry Blake, Trapper John McIntyre, and Frank Burns are all married men who are having affairs while serving in Korea.
Conversely, in a season 6 episode B.J. Hunnicutt cheats on his wife and feels terrible as a result.
It's also revealed that Major Houlihan's husband has been cheating on her when he sends her a letter he intended for his lover.
The subplot of one episode dealt with a man furious to receive a letter from his wife admitting to sleeping with another man and begging forgiveness. BJ is less than impressed when he finds out that the man himself has a Korean girlfriend.
The Tudors has a lot of this. Henry cheats on his first three wives. Katherine of Aragon takes it stoically, Anne Boleyn blows up over it, and Jane Seymour actually doesn't seem that bothered - she is perhaps remembering that Anne Boleyn got her head cut off and is therefore being cautious. Plus, is was perfectly normal and socially acceptable for a man in Henry's position to have multiple mistresses, so whether it would actually be considered as "cheating" by the standards of the time is debateable. Needless to say, the same rule did not apply for married women. Wife no. 5, Katherine Howard cheats on Henry with Thomas Culpepper, and is executed for it. Part of what ruins the Charles Brandon/Princess Margaret marriage is that Charles can't be faithful. Both of the gay couples on the show involve married men, so they're cheating on their wives. Technically Ursula Misseldon was cheating on her fiance with Francis Bryan and Henry. Anne Stanhope cheats on Edward Seymour twice, first with Francis Bryan and then with his own brother, Thomas (though we never know for sure if Edward knows about the second). She also flirts with the Earl of Surrey, but Edward seems to have been encouraging that one... It's also inverted, since Anne Boleyn gets executed for adultery when she didn't cheat on Henry.
What's really wild is that 90% of this is Truth in Television in fact if anything there was more screwing around in Henry's court than the show portrays.
In The Mentalist, Bosco's utter devotion to Lisbon is played completely sympathetically, even though he is married. It may help that Lisbon is stoutly convinced that Bosco doesn't love her back, and actually cites his wife as a reason. Turns out she was wrong. Which she discovers as he is dying. Oh, God. That was a Tear Jerker.
Jimmy McNulty from The Wire is the personification of this trope.
Bunk to a lesser extent.
Kima gets there too, after spending too much time around McNulty
Danny Messer on CSI NY during the Will They or Won't They? period of his courting Lindsay. A boy in Danny's care was shot, and mourning and comfortsex with the kid's mom ensued.
A Valentine's day episode has a wife killing her diabetic husband because she thought he was cheating on her. Turns out he was planning a surprise vacation for their anniversary and the woman he had been talking to was a travel agent
Subverted on CSI: Crime Scene Investigation. It did look like Doc Robbins' wife was cheating, but it turned out to be a set-up involving a disgruntled, arguably Ax Crazy guy looking to off a genealogist who pulled up some dirty family secrets on him and Mrs. Robbins just happened to be said genealogist's next client.
Played straight in Brass's backstory. It was a double cheat, since they were both cheating on each other. Ellie resulted from the wife's affair.
Generally speaking, many of the gangsters are noted adulterers.
Mad Men: Don Draper is the quintessential cheater. His wife is very anxious, feeling that their relationship is not right even before she finds out the truth.
In Season Three of Buffy the Vampire Slayer, Willow and Xander cheat with each other on Oz and Cordelia, respectively. They don't have sex, but these are high school relationships, so making out is as far as anyone is going. They are eventually discovered. Cordelia breaks up with Xander, and temporarily leaves the Scooby Gang. Oz eventually forgives Willow. Both Willow and Xander remained sympathetic, mostly because they were very distraught over their powerful attraction to one another.
Oz cheated on Willow in Season Four with fellow werewolf Veruca. This time, sex was involved, but because Oz was a werewolf when the cheating took place, and not in control of his actions he was plainly and visibly struggling with mating urges the wolf inside him was feeling toward the wolf in Veruca, which partially explains why he had to bring her into the cage with him. The other part is that she would have gone on a killing spree otherwise. Before the normal drama could play out, the plot got supernatural, and Oz killed Veruca to prevent her from killing Willow. Oz went away, and Willow went gay.
In the episode "Family 8108" from Cold Case, it appeared that the Japanese father cheated on his wife as he caught kissing the school teacher who worked at the internment camp. However, it was averted as that was all they did and the husband told the teacher right away that he loved his wife and the teacher understood. But nevertheless, it spurned his son to join the army out of spite for his father.
Finely subverted in The District: The mayor is spotted having discreet meetings with a young Asian-looking woman. Turns out she is his illegitimate daughter from when he was serving in Vietnam.
In the fifth season of Charmed, Paige briefly dated a guy until she found out he was married with two little children.
This is the basis for the iCarly / VictoriousCrossover. Carly's boyfriend Steven has been cheating on her with Tori and vice versa, and enjoying it. Carly ends up finding out the truth and teaming up with Tori to humiliate the cheater live on the internet.
The show starts off with Will & Emma falling in love despite Will being married to (and thinking he's having a baby with) Terri. Before long this becomes not only Will as a cheater but also Emma, as she's dating & then engaged to Ken. They almost-kiss multiple times and there are scenes of Terri & Ken being jealous of Will & Emma. It is an emotional affair.
In season 1, episode 2, Finn kisses Rachel despite the fact that he's dating Quinn. Rachel kisses him in 1x05 when he's still dating Quinn plus he now knows she's pregnant. Quinn feels very jealous of any times Finn & Rachel seem close throughout all of the first half of season 1 while she is still dating Finn.
In episode 1x14 Will cheats on Emma by having a heavy make-out session with Shelby.
Both girlfriends Finn has had have not only cheated with him, but cheated on him with Puck.
Quinn cheats on a second boyfriend of hers as well- she cheats on Sam with Finn in season 2.
Brittany cheats on Artie with Santana - because Santana convinces her its not cheating because the plumbing is different.
In season 3, Beiste feels she is being cheated on by her boyfriend, Cooter. He just thought she wasn't that interested in him and was moving on.
In one of the more dramatic instances, Blaine cheats on Kurt in season 4 and it breaks them up and leads to both of them feeling insanely devastated.
In 4x14, even though Ryder is close friends with Jake, he kisses Jake's girlfriend Marley. She does, in the next episode, tearfully confess this to Jake in addition to admitting "and I let him".
The writers of Glee have gotten a lot of flack for seemingly thinking that the only way to create drama or break up a couple is to have the involved parties cheat on each other. Many fans feel that the cheating storyline has gotten very old because they use it time and time again, when there are so many other interesting ways a relationship can fall apart.
New Girl: As part of the show's premise, Jess returns home early in hopes of surprising her live-in boyfriend, only to find him cheating with another woman. This leads to her breakup and search for a new apartment.
Niles' wife Maris on Frasier with their marriage counselor of all people, and just a few scant weeks after she had undeservedly managed to cajole Niles into returning to her, to boot. This was particularly devastating to him, since he himself had struggled with an attraction to Daphne for years but always quashed it and remained steadfastly faithful while he and Maris were together.
Frasier's wife Lilith also cheated on him back in Cheers right before she dumped him for the man she was cheating on him with, which caused Frasier to nearly commit suicide and earned her several years of unrelenting wrath from Niles and Martin. In a very different spin on this trope, Hester Crane also cheated on Martin decades ago, but the two of them overcame the incident and had an otherwise very happy marriage.
Joan suspects her boyfriend Adam and her best friend Judith are having an affair when she witnesses them hug. Turns out it was all platonic.
Joan also gets jealous when he hires an attractive, perky blonde assistant for his work and ends up being the first to actually cheat on him - she is a senior in high school and kisses her college-aged tutor, Roger. She feels very guilty and confesses. They don't break up over this.
Adam ends up cheating on Joan by sleeping with Bonnie, an artist he meets. He thought it could just be sex. Joan wasn't sleeping with him for some reason (she wasn't ready to lose her virginity yet) and his excuse is that he was just that desperate for sex. They break up near the end of the series over this.
A more borderline case on the show would be when Will has an emotional affair with his boss. He didn't really cheat on her. He almost did.
Necessary Roughness starts off when Dani finds out that her husband has been having affairs right under her nose and has even been using the guest bedroom in their house for it. She promptly kicks him out of the house and files for divorce.
The romantic story arc in Hannah Montana between Miley and Jake Ryan comes to an end in season four, when Oliver takes a photo of Jake nibbling the ear of a female co-star during a movie shoot. Oliver and Lilly try to hide this from Miley, but she finds out eventually while shooting a Christmas special she and Jake from a boy extra who accidentally reveals the secret when Lilly and Oliver are talking to her. She retaliates by beating up Jake on camera.
A season 3 episode has Oliver practice saying "I love you" to an unconscious Miley in the hospital (she injures herself in a skiing accident in the episode's open); Miley's subconscious self overhears him in an out-of-body experience and thinks Oliver is cheating on Lilly with her. Miley's disgust for Oliver (mixed with Lilly's overbearing sadness at finding out, and Miley's flattery that Oliver is attracted to her) are Played for Laughs for much of the rest of the episode, along with Miley's attempts to encourage a still-emotional, makeup-streaked Lilly to find another boyfriend at the beach. Naturally, Lilly and Oliver reconcile, and are upset with Miley.
In Good Luck Charlie this happens to Teddy. It turns out Spencer had told Teddy that Skyler was his cousin and vice versa, it doesn't end well for him. Teddy eventually forgives Spencer and they get back together, while Skyler gets together with PJ, Teddy's brother.
The infamous Love Triangle of Arthur, Guinevere and Lancelot is dealt with in a unique way in Merlin. Gwen and Lance were each other's First Love, but because of circumstances they never had a chance to get together properly. Guinevere eventually falls in love and gets engaged to Arthur, only for Morgana to send a Brainwashed Lancelot to the celebrations in order to confuse and upset Guinevere. When Gwen doesn't react to Lancelot's return, Morgana ensures she is Mind Raped by an enchanted bracelet and ends up caught in a compromising situation with him. Even Guinevere doesn't understand why she acted the way she did, and the real tragedy is that she admits that she did once have feelings for Lancelot, as did Lancelot for her - but the audience knows both were too noble and too devoted to Arthur to ever even consider acting upon them.
In How I Met Your Mother, Ted's highschool sweetheart Karen, whom he dated off and on throughout college was most notorious for cheating on him constantly, fueling their endless cycle of break-ups and make-ups. This caused Lily and Marshall to develop a vendetta-like hatred of her and give Ted a lot of flack for repeatedly taking her back.
In season 3, Robin cheats on her current boyfriend with Ted.
In season 7, this happens to both Barney and Robin who cheated on their respective girlfriend and boyfriend at the time.
The nightmarish situation was nicely summed up as "Well, I'm stuck on a boat with the guy I cheated with, the guy I cheated on, and the girl who the guy I cheated with, cheated on, so no, I'm not doing so good."
Way before he met the gang, Barney was deeply in love with his long-time girlfriend Shannon who he would have married if he didn't find out she cheated on him with a suit-wearing jerk, prompting Barney to start wearing suits.
Subverted when Ted thinks that his father is cheating on Ted's mother. It turns out not to be cheating because Ted's parents have been divorced for a while and they failed to inform Ted of the fact.
This was a fairly prominent part of Jackie and Kelso's relationship in That '70s Show. Note nearly all of the affairs are one night stands.
Kelso would cheat often and freely without Jackie ever suspecting a thing. One affair in particular - the one with Eric's sister - becomes more a relationship than just a one night stand like the others and is the one he is caught in.
Jackie's turn: Later, she kisses The Cheese Guy, which Kelso fails to see the irony in. This is lampshaded:
Kelso: Alright, look. Jackie, here's the deal, you cheated on me.
Jackie: You used to cheat on me all the time.
Kelso: Yeah? Well, yeah. But you cheated out of hate, and I cheated out of joy.
Later still, Jackie and Hyde start dating and Hyde thinks Jackie is cheating on him and in response actually does cheat on her.
Also Milah fell in love with Captain Hook and left her husband and young child to be with him, faking a kidnapping from Hook and leading her family to assume she was dead. When her husband found out she was not only alive but cheated on him with Hook, things did not go well with her.
Infidelity and people's attempts to cover up or get revenge for said infidelity is a frequent plot point in Person of Interest.
China Beach had several characters who considered themselves "geographically single."
The biggest one known to fans is between Ross and Rachel. After they get into a huge fight, they decide to "go on a break" (meaning spending time apart but the relationship is still on). Ross interprets this as "fool around with the copy machine girl" in order to cope. When Rachel finds out, it leads to the biggest heartbreak between the two of them and a strained relationship for the next several seasons.
A smaller example between Joey's parents. Joey discovers his father has a mistress and gets angry at him, demanding that he go make things right with his mother. The following day, Joey's mother stops by and gives him an earful for what he did because she was actually happier that her husband was cheating, which in turn made him happy since the two of them were miserable being faithful to each other.
In one episode, Chandler dates a beautiful woman and is very surprised when he finds out she's married. She insists that her husband is ok with it, and they continue dating no-strings-attached. Chandler first likes it, being afraid of commitment, but gets uncomfortable later, especially when he finds out she has another lover beside him. They break up.
One rather big arch involved several instances with lots of broken hearts. Chandler fell in love with Joey's girlfriend, and while Joey was mostly casual about his relationships, he decided to keep Kathy around for a bit longer and considered going steady with her. She liked him well enough, but had deeper connection with Chandler, who was absolutely crazy about her and suffered a lot. They kissed once, and when Joey found out, he got really upset. Kathy then wanted to break up and not pursue a relationship with either because she felt it would separate them and would be too hard on all. When Joey found out how much Chandler cared for her, he green-lit the relationship. Chandler and Kathy were happy and perfect together for some time, but later Chandler became insanely jealous of Kathy's colleague from theatre. They had a fight and she slept with said colleague. She thought they were broken up; he thought it was just a fight. He was devastated, going into Heroic BSOD for the next few episodes. Love Hurts.
In Farscape, Chiana cheats on D'Argo with his son. She is presented somewhat sympathetically when she explains that she did it because she couldn't handle the amount of commitment D'Argo was trying to force on her. Understandably, Jothee leaves the ship and the relationship between D'Argo and Chiana remains strained for quite some time.
Monday Mornings: Dr. Tina Ridgeway's marriage is a mess and falls apart. It's not absolutely crystal clear whether she slept with her colleague before or after she and Mark broke up, but she probably did. When she tells her husband that she's having an affair, he files for divorce and blames her for destroying their marriage. However, he blames her lover even more and sues him.
As you might expect, cheating songs are quite common in Country Music.
"The Night Will Only Know," another Garth Brooks song, is about two people married to other people who have a one-night stand and end up witnessing a murder.
Similarly, Lefty Frizzell's "The Long Black Veil" features a dead narrator who was falsely accused of a murder and chose to hang for the crime rather than reveal his alibi (he'd been with his best friend's wife on the night in question).
"In Another's Eyes", a duet by Garth Brooks and Trisha Yearwood (though this one is more about two people who are trying not to cheat).
"Here's A Quarter (Call Someone Who Cares)" by Travis Tritt.
"I May Hate Myself in the Morning" by Lee Ann Womack.
"Cheater, Cheater" by Joey + Rory, wherein the female asks why her man was cheating.
...same with "Cheatin'" by Sara Evans.
"Does He Love You" by Reba McEntire and Linda Davis is sung by the wife and the 'other woman'.
Also by Reba: "Whoever's In New England" and "It's Your Call." Both involve a wife aware of her husband's cheating - the first says, "You'll always have a place to come back to, when whoever's in New England's through with you," and the second has the lover calling the house.
And again by Reba: "Little Rock," which involves a woman deciding to cheat because the marriage fires have dwindled.
The Murder Ballad "Matty Groves" revolves around the titular character spending the night with a nobleman's wife, only to have the nobleman himself come home early (thanks to a tattletale servant). The nobleman kills Matty Groves right away, then asks his wife which man she prefers. When she defiantly answers "I'd rather a kiss from dead Matty's lips than you or your finery," he kills her too.
"Lips of an Angel" by Hinder (Covered Up by country singer Jack Ingram).
"Lying" by Amy Meredith
Glasvegas' "It's My Own Cheating Heart That Makes Me Cry," which never quite makes its mind up as to which side it's on (the relationship is certainly screwed, but it's tricky to tell whether it's because he cheated or because he's completely petrified about her cheating, or both, or neither, or...). In a cross between Soundtrack Dissonance and Foreshadowing, it appears in Skins when Naomi And Emily finally get together and Thomas And Pandora reconcile in S3's penultimate episode; by the time they get two episodes into S4 though... well, see the Skins list in Live Action TV above.
"Revenge Is Sweeter (Then You Ever Were), Everything I'm Not, Did Ya Think" - The Veronicas
"All I Have" - The Veronicas from the perspective of the cheater
"The Long Black Veil"
"Over the Hills and Far Away," a folk ballad by Gary Moore covered by Nightwish and Thyrfing, has a similar plot to "The Long Black Veil," with the protagonist unable to confess his innocence to a robbery because of spending the night with the wife of his best friend and having to go to prison.
Two Wham! songs deal with cheating in various ways — "Last Christmas" features a protagonist who's just broken up with his partner because his partner cheated on him, but "this year to save" the protagonist "from tears" on Christmas, he'll give his heart "to someone special". Then there's "Careless Whisper", a song where the protagonist is the cheater and is feeling extremely guilty about having done so. ("Guilty feet have got no rhythm", "Should've known better than to cheat a friend/The wasted chance that I've been given", etc.)
Duran Duran's 1981 song "Careless Memories" deals with the protagonist's anger at having found out about his partner's cheating on him and how disgusted he is with said partner. He even regards the exchanges of love and devotion that went on during the relationship as "careless memories."
"The Other Man" by Canadian band Sloan, based on Chris Murphy's relationship with Feist, who was also dating Andrew Whiteman of Broken Social Scene at the time.
"Save It For The Bedroom" by You Me At Six is another example where the singer is the one cheating: "Keep your hands to yourself,these lips belong to someone else..."
"Escape (The Pina Colada Song)" is a strange example. A man decides to cheat on his wife with an unknown woman who posted an ad in the paper looking for romance. He arranges to meet with her by sending a reply message to the paper, and when he reaches the rendezvous, he discovers that the woman he plans to cheat on his wife with is... his wife. Strangely, catching each other trying to cheat on each other revitalizes their marriage instead of destroying it.
"You Know I'm No Good" by Amy Winehouse is sung from the cheater's perspective- she loves her partner but has trouble staying faithful. "You say when we marry 'cause you're not bitter/There'll be none of him no more/I cry for you on the kitchen floor"
Backstreet Boys's The Call is about a man who cheats on his girlfriend during a party. Made even more obvious in the music video, where the five band members are chased around by a beautiful woman who seems to be the incarnation of the guilt the narrator feels.
In Pepe Deluxé's Queen of the Wave, the hero Zailm's downfall comes about because he begins an affair with Lolix, "leading to duplicity with Anzimee [his wife] and a strange blindness to the wrong he has committed."
"It Wasn't Me" by Shaggy is an interesting example. The song's protagonist was just caught cheating and goes to Shaggy for advice. Said advice ranges from hoping she'll forget, to flat-out denying, to hoping she doesn't have a gun. In the end the protagonist decides to ignore all of Shaggy's advice and man up to what he's done, showing that there may still be hope for their relationship.
"Smile" by Lily Allen is about a woman who takes a systematic revenge on her cheating ex, which includes getting him beaten up, putting laxative in his coffee and having his stuff in his apartment destroyed, including his precious LPs.
Myth And Legend
Pick a Greek myth. Any Greek myth. From Zeus (married to his sister, Hera) seducing or outright raping everyone, male or female, to Aphrodite (married to long-suffering Hephaestus) and her relationship with Ares, to Paris eloping with Helen of Troy, the wife of Menelaus... no wonder there were so many children of questionable parentage running around.
Over in Norse mythology, we have Sif- married to Thor, and though it was never confirmed to be true or false, Loki at one point accused her of sleeping with him ("Only I know, as I think I do know/Your love besides Thor/And that was the wicked Loki!"). She says nothing, and then another goddess starts talking. And in an earlier myth, Odin (in disguise) said to Thor the equivalent of "Hey, no need to hurry home to your wife or anything, because she's got someone else to keep her bed warm!" Both times, Thor was understandably pissed.
In Japanese myths, the flower goddess and Mount Fuji patron Konohanasakuya-hime was accused of cheating on her husband Ninigi-no-mikoto, the grandson of Amaterasu, since she announced her pregnancy the day after their marriage. Her reply was to lock herself in a burning hut to give birth, saying that her baby wouldn't be hurt if they truly were the offspring of Ninigi. She was proved right... and gave birth to healthy triplets (Hoderi, Hosuseri, and Hoori).
In Street Scene, Mrs. Maurrant is Cheating With The Milkman, who the neighbors point out is himself married with two children. Her daughter Rose is aggressively propositioned by her boss, Harry Easter, who tells her that his wife doesn't need to know about anything he's planning.
In Chicago, Roxie is cheating on her husband Amos with Fred Casely, a married man whom she shoots to death at the start of the play; Amos grudgingly stands by his wife until she decides to draw sympathy in her murder trial by pretending to be pregnant. The musical also has Cell Block Tango, which contains a minimum of four affairs, depending on how you count it.
In Rodgers And Hammerstein's Allegro, when Joe moves to the big city, it turns out that his wife is cheating on him with Lansdale, the benefactor of the hospital he works for. He finds this out from Mrs. Lansdale.
In Noh theatre, the bridge princess youkai is the eternally-jealous spirit of a noblewoman whose husband cheated her. Depending on the retelling, she either murdered or cursed him, and the gods turned her into a bridge-haunting youkai as punishment.
In A Shot In The Dark, it gradually emerges that while the first suspect was cheating on the victim with the master of the house, his wife was having an affair with his best friend. Sevigne helps make the point that while adultery "is not illegal—it's not even unpopular," murder is "certainly illegal, and probably unpopular."
You can accidentally (or not) do this in Dragon Age: Origins. Picking any of the more flirtatious dialogue options with a potential Love Interest will make them think you're in a relationship. If you're already involved with someone, you'll eventually be forced to choose; losing approval with the one you give up.
If a female PC woos Alistair and later arranges for him to marry Anora, they can stay together. Alternatively, if a female human noble marries Alistair, but romanced Zevran, the same situation can happen in reverse. In both cases, it's an affair out of a political marriage, rather than a romantic relationship, which has different implications.
As a male Warden, you can exploit a glitch that allows you to be in a relationship with both Morrigan and Leliana, even after you're supposed to have chosen only one of them.
Chandra volunteered her eternal love to Karim if he could get his hands on a certain treasure. This one ends tragically for both of them. For Chandra, she fell in love with another guy, hence the trope, only to infuriate his beloved and get herself killed. She makes it to Karim in time to convince him not to pick up the artifact, and instead to guard it for however long it takes. She confesses the error of her ways, and he lets loose upon her as soon as he learns the truth before volunteering his services in guardianship to the artifact. "The things I do for love..."
In Metal Gear Solid, Meryl's mother cheated with her husband's brother, Colonel Campbell, resulting in Meryl.
You can do this in Mass Effect 2, if you romanced someone in both games. There are ramifications for being unfaithful in the third game, though the nature and severity of the consequences depends on the characters involved. Ashley takes Shepard cheating on her with Tali very well, for instance, and both Ashley and Kaidan essentially declared any romance with Shepard over on Horizon in the second game, so Shepard wasn't being unfaithful. During the Lair of the Shadow Broker DLC Liara eventually gets over it if you wound up with someone else, but she's clearly still upset about it if you were with her in the first game. And since the romances started in ME2 were consummated right before a suicide mission, if the player chooses to end things in the third game, it's understood.
Jacob Taylor ends up in a relationship with Dr Brynn Cole (and has a child with her) even if you romanced him as a female Shepard.
In Grand Theft Auto IV, Mallorie Bardas cheats on her boyfriend, Roman, with Vlad Glebov. Roman is OK with it, but his cousin, Niko, is not, and kills Vlad.
Roman himself states sleeping with different women during the course of their relationship. While he eventually stops cheating on Mallorie, his favourite activity is still visiting strip clubs.
This is Battler's reason for disowning his father Rudolf in Umineko No Naku Koro Ni: almost immediately after Asumu's death, Rudolf engaged himself to his business partner Kyrie, who was already pregnant with Ange. And then it turns out that Battler is actually Kyrie's son, having been Switched at Birth with the child of Asumu, the woman he thought was his mother, after her child was stillborn, and Rudolf did this to cover up the affair he had with Kyrie without either woman knowing.
Persona 3 requires you to max out various "social links" to get the best ending. Unfortunately, all of the social links with girls your age (or with most of the guys if you pick the female protagonist in the PSP remake) involve dating them and imply sleeping with them when you max them out. This trope is played straight in that the girls will get jealous if you date multiple girls at once, but is subverted in the end because once you reach the ending of a social link, you're free to go after another girl with no threat of breaking the link.
There's also a great scene added to Persona 3: FES where all the girls you are dating at school almost figure out what's going on and almost corner him while setting up for the school festival.
The female protagonist can actually get two party members to bicker over her in combat if she maxes out social links with them. Oddly enough, she has an easier time cheating because she doesn't have to worry about reversed S.Links from running around like the male character does despite no one ever mentioning it outside of the above mentioned bonus scene from FES.
Persona 4 uses the same social link mechanic, but you're given the option of keeping your friendships with the girls platonic and pursuing only one or no girls. However, you also have the option of sleeping with them all if you do it right.
The game In The1st Degree has this happen. To start, you have Zachery Barnes and his wife Yvonne Barnes, as well as James Tobin and his girlfriend Ruby Garcia. Yvonne has been very focused on her career, leaving Zach unhappy. Tobin has slept around, and even told Ruby that he is only interested in getting into her pants, leaving Ruby unhappy. As a result, Zach and Ruby had an affair. There were reprecussions from this. First, Zach wrote a love letter to Ruby admitting that he loves her, but he is convinced that the affair should not have happened. Second, Tobin found out about the affair and was furious over it, giving him one more reason to murder Zach. Third, Yvonne found out about the affair, and brought the gun from Zach's cousin Daryl Barnes to the art gallery to threaten her husband. Yvonne and Zach did have a long talk and may have planned to give their marriage a second chance. Yvonne did tell Zach about the gun, but he laughed and decided to have it locked in his desk until he can return it to his cousin. Now that is drama!
In L.A. Noire there's a few cheating couples in the various missions but more prominently is Cole's affair with Elsa which Roy outs, splashes around the newspapers and causes Cole to get demoted from Vice back to Arson. Adultery was illegal in 1947, when the game was set.
In Culpa Innata, the citizens of the World Union are encouraged to avoid romantic attachments. Marriage (or rather "nuptial agreements") are not permissible for Union citizens. They are, however, allowed to have primary sexual partners, which means absolutely nothing from a legal (or personal) standpoint. During the game, one of the potential Union citizens is an Indian immigrant, who reveals that he has cheated on his wife on several occasions and doesn't feel guilty about it (he's also a sociopath), while his wife never did because of the social Double Standard in Indian culture.
You almost get away with this in Rune Factory Frontier by asking another girl on a date and then proposing to her with the wedding bouquet, which is the item required for marriage of any girl not playing a part in the storyline, and can be synthed multiple times. Presenting the bouquet when prompted will lead to a short scene where the dated girl calls you out on it, and while this happens your wife approaches and catches you red handed. After two emotional lines depending on the wife she then proceeds to (understandably) beat the everloving shit out of youwhile the nowSHAKINGcamera pans to show Whale Island, during which the gloriously deadpan line "[Wife's name] is angry." appears on the screen accompanied by a "Don't try this at home" seconds later. Raguna's cries of pain illustrate as audio during the whole thing, and then the camera cuts back to your date vanished and you collapsed on the floor. She leaves you with exactly 1 HP and 0 RP, and her Love and Friendship gauges are reset tozero. Even more jarring if you married someone pacifistic, like Tabatha or Lara.
The issue of just what constitutes cheating was raised in Friendly Hostility. Fox is devoted to his boyfriend, Collin, but his bisexual colleague Derringer springs a kiss on him, just to see what it's like. Fox is outraged. However, he's even more outraged when Derringer questions Fox's kissing skills... and so Fox kisses him again to prove his aptitude. On the Friendly Hostility blog (doubling as The Rant), fans varied between "kissing is cheating" to "it was just a kiss" to "kissing usually counts, but there was no affection behind it, so it doesn't." Some fans even suggested that it might be good for Fox and Collin to break up following this incident, since they didn't think it was psychologically healthy for Collin to only have one romantic/sexual partner in his lifetime and he needed to "see other people" rather than remain exclusive to Fox.
Nerf NOW!! had Sniper-tan cheating on Pyro with the Spy, after an argument. Spy later produced ambiguous evidence that Pyro had had an affair with Engie-tan. Subverted in that nether of them were caught... at least by each other.
On Daria, Daria and Tom kiss while Tom is dating Daria's best friend Jane.
It's implied that both Brittany and Kevin have cheated on each other at various times, though this is Played for Laughs.
On King of the Hill, Nancy cheated on her husband Dale for years with John Redcorn. Despite being a paranoid conspiracy nut, Dale never even figured out that their clearly Native American son wasn't his (though he does believe he was conceived by alien in-vitro, since he wasn't even home around the time of conception). The twist is that everyone else knows about the affair. They all just don't tell Dale because they want to spare his feelings and they know Nancy still loves Dale despite her cheating.
Also, Buck Strickland cheats on his wife. Quite a lot, in fact.
In fact, so does his wife, as evidenced in an episode where she tried to come on to Hank.
One episode reveals that Mona was cheating on Abe around the time of Homer's conception, leading to questions about whether or not he was the father. It turns out he really is.
Homer's half-brother Herb Powell is the product of an affair between Abe and a woman who worked at a carnival.
On South Park, Butters' dad cheated on his mom with numerous gay men, leading her to go crazy and plan to kill herself and Butters.
Cartman was conceived from an affair between his mother and Jack Tenorman, Scott Tenorman's father. Also implied between her and Kyle's dad Gerald, another potential father, though in another episode he's apparently fine with his wife sleeping with Chef, implying they may have an open marriage.
A really messed up example occurs in the Ren and Stimpy episode, Ren's Brain. Ren, who was at the time a disembodied brain and unaware of it, comes home from work to find Stimpy sitting with Ren's body. Cue Ren's brain having a Freak Out, accusing Stimpy of " Entertaining another chihuahua", calling his body a "homewrecker", all culminating in the viewers' heads exploding, then the Earth itself explodes and Ren's brain is seen floating off into space yelliing "YOU EEDIOT!"
In Kick Buttowski Kendall is technically cheating on Ronaldo with Kick, with her multiple moments of flirting with and kissing him while still officially dating Ronaldo. It's mostly not portrayed as a bad thing, however, since earlier episodes established that it's Kick she's always had her eye on.
Moral Orel: Principal Fakey is cheating on his wife with Nurse Bendy (who's fairly indifferent to the whole affair). He feels guilty about it (and Reverend Putty shows off more jealousy than advice), so Orel tells him that as an authority figure he can do no wrong . . . so he continues the affair guilt-free in denial. This does NOT end well.
Another example more relevant to the plot: Bloberta cheated on Clay with Daniel Stopframe, which resulted in Shapey. And of course in the present, Clay seems to have something going on with Stopframe. Who he then cheats on with Ms. Censordoll.