A character has their lust aroused by someone besides their lover, prompting to act out that lust on said lover.
In many works, for a partnered person to be aroused by any Lust Object besides their partner is a forerunner to danger, or at least high drama. But sometimes a character will perform a kind of lust judo and redirect their heightened libido in a more acceptable direction.
The situation need not be an actual encounter with a person, it could for instance be hearing a story, including one involving their partner but not themselves. However, things being as they are, the math is often that a man hearing about his female partner's experience with a woman will lead to retargeted lust, as will a woman hearing about her male partner's early experience with another woman, but other combinations will just lead to trouble.
Given that many partnered people live their lives surrounded by potentially arousing people, images, and ideas, and yet somehow most of them remain mostly monogamous most of the time, it is probable that this phenomenon is if anything more common in Real Life than in fiction.
Does not include circumstances such as a couple watching pornography together, which is more passive, planned and "safe" and should probably be its own trope.
If the Lust in question leads to solitary activity (or at least that without another living being), then it's more of an I'll Be in My Bunk moment.
- In It's a Wonderful Life, Bert the cop, after watching the slinky Violet walk down the street, says: "Think I'll go home and see what the wife's doing."
- In Girl with a Pearl Earring, after the married artist and his model have an unplanned steamy moment, they separate. She goes off to have her way with the butcher's boy who always liked her, while he has sex with his wife.
- In The Tailor of Panama, a married woman has tense and increasingly sexy banter with the spy who may be endangering her husband. After he leaves, she has sex with her husband and he with his new girlfriend.
- In Wild at Heart, just hearing about one of her lover's past encounters makes a young woman, in her words, "hotter than Georgia asphalt."
- It seems like the wife in Eyes Wide Shut was partly trying to get this reaction from her husband when she tells him her fantasies about running off with a sailor, but instead it plunges him into turmoil and doubt. So she eventually takes the direct approach.
- In Basic Instinct, having been thoroughly taunted and titillated by murder suspect Catherine Trammell, Detective Nick Curran more or less forces himself on his on-again/off-again girlfriend, police psychologist Beth Garner. There's a similar scene in the sequel with Dr. Michael Glass, Catherine's own psychologist and his girlfriend.
- Implied in Body of Evidence. A scene of sexually charged dialogue between Frank and Rebecca is followed by a love scene between Frank and his wife, Sharon. Sharon remarks that Frank seems different somehow.
- A Most Wanted Man (2014). Irna teases fellow intelligence agent Gunther that he fancies the pretty blonde lawyer they're tailing. He replies that she's not his type, Irna is. Then they Fake-Out Make-Out as the lawyer passes their position, with Gunther eyeing the lawyer while he's kissing Irna.
- Kinda what happens in "Victims of Circumstances" novella from the first volume of Labyrinths of Echo: Max and Melamori, who kept having rather moist dreams about each other (despite not really being in a relationship when awake), fall prey to some Wild Magic and wake up in the same bed one morning. After all the yelling and name-calling, Melamori storms off, saying she'll go visit the local Red Light District and have sex with a total stranger, just to spite him. Before long, he ends up in said district, as well. Because the "brothels" in Echo work differently from our world (it's more of a blind dating than prostitution), they end up being accidentally matched together, anyway—which only complicates their relationship even more.
- Discussed on more than one occasion in the Dragonriders of Pern world, when two dragons want to mate, but their riders don't want to do so with each other (Incompatible Orientation, a possible incestuous pairing, one or both riders are in committed relationships, or they simply hate each other). The solution is for each rider to stay near his/her preferred partner during a flight, or find a willing third party for the occasion.
- Gone with the Wind: Scarlett is downright terrified when Rhett calls her out on this:
Rhett: Don't you suppose I know that you've lain in my arms and pretended I was Ashley Wilkes?
- In the first episode of Coupling, a woman doles out (possibly fictitious) stories of her past encounters with women in order to keep her boyfriend interested.
- In Married... with Children, it happens several times that Al, turned on by other women, has sex with Peggy (something he normally detests). In the episode, "Live Nude Peg", the "other woman" is actually Peggy, who performs a striptease in Al's favorite strip club wearing a veil.
- On The Big Bang Theory, Howard expresses arousal at some girl in a gym they're visiting; Penny is disgusted, so Howard explains that Bernadette 'doesn't care who gets my engine running, as long as I park it in the right garage'. Penny can't believe he's with Bernadette.
- An episode of Roseanne in which Dan meets his old high-school girlfriend, then goes home and has amazing sex with Roseanne. When Roseanne works out the chain of events, she gets jealous.
- Desperate Housewives had this happen when the Scavos got a (young, pretty) nanny and Tom accidentally saw her naked. Lynette figured it out fairly quickly.
- Salem: When Mary starts to invade John's dreams, it reignites his passion for her. To try to stave off those feelings, John goes to the whorehouse and hires five separate whores.
- Buffy the Vampire Slayer. Spike has sex with Harmony while thinking of fighting Buffy (in a later episode he gets her to dress up as Buffy before sex). Then he has a Buffy Sex Bot built for him.
- Angel. Lilah dresses in Sexy Spectacles to jointly seduce Wesley and mock him for being in love with the bookish geek girl, Fred. As they're ripping off each others clothes to have sex, Wesley interrupts their passion to insist she keep the glasses on. The expression on Lilah's face suggests he may as well have said Fred's name during the height of passion, it stung that much.
- The Bold and the Beautiful:
- Ridge comes home from a long day of work, picks up his wife Taylor and pushes her against the wall. Following the Sexy Discretion Shot, we fade in to find them lying in bed, drenched in sweat and utterly exhausted. All's well until Taylor starts asking Ridge about his day and he mentions having had a run-in with his ex, Brooke. It doesn't take long for Taylor to realize that there's a good chance Ridge was fantasizing about Brooke the entire time he and Taylor were going at it.
- Bridget is devastated to overhear her husband Deacon confessing to Amber that not only did he only marry her to hurt Amber and make her jealous, but that he sometimes fantasizes about Amber while in bed with Bridget.
- ER. Doug dares Carol to deny that she does this when she's with her boyfriend, in front of said boyfriend. It gets him punched for his trouble. However, a fanfic has them later admit that they've both done this about each other while with others.
- One Life to Live:
- After fighting with ex-boyfriend Cord, Blair runs home and seduces husband Todd (she dumped Cord so as to marry Todd for his money). Todd suspects the real reason behind her actions, but doesn't say anything. It actually marks a turning point in their relationship and the development of legitimate feelings for each other.
- Blair admits to having done this a few years earlier in a virtually identical situation—after dumping Max to marry Asa for his money, she consoled herself in her loveless marriage by pretending that Asa was Max during what she claims were marital rapes.
- An episode of Yes, Dear has Jimmy and Greg visit a strip club and come home to have sex with their wives. An uncaring Christine mentions this is why their husbands were so enthusiastic in bed to Kim - who is furious.
- Million Yen Women: Living with the women causes Shin to get horny from time to time. Fortunately for him, the million yen of monthly rent he gets from the women enable him to afford a High-Class Call Girl.
- Sisters Teddy hooks up with the man who received her late husband's heart. He gets increasingly uncomfortable with her efforts to turn him into a Replacement Goldfish, finally asking her, "Who were you really in bed with last night? Him or me?"
- In Miss Saigon, at the end of the song "I Still Believe", Chris grabs his wife Ellen and kisses her passionately. . .having just awakened from a nightmare about his lost love Kim. It's not hard to imagine that he's transferring his feelings for one woman onto the other (especially the handful of times Ellen has been played by an Asian woman rather than a white one).
- South Park: A bout of impotence keeps Gerald from having an erection until a couple of women come into his office to complain about global warming which is making them get skin cancer. They take off their clothes to show him, he gets excited, and runs home to have sex with his wife.
- The Simpsons: In the episode "Feelin' Frisky", Homer hints at a heightened desire for Marge after watching women's volleyball on ESPN.
- One episode of American Dad! has Stan taking a stripper named Tanqueray into the house so he can help her set up a business. When he goes to wake her up one morning, she accidentally flashes him getting out of bed, and Stan tells her to be more careful. He then casually tells her to meet him in the car in fifteen minutes. He then walks out and, in the same tone of voice, tells Francine to meet him in the shower immediately.
- Actress Mariette Hartley has said that during production of "No Place To Hide" (1981), she would get stirred up filming love scenes with co-star Keir Dullea, then go home and jump her husband. At the end of filming her husband told her, "You know, I'm going to miss Keir..."