In Dragon Bones, it is mentioned that Ward's father cheated on his mother with her half-sister. Ward suspects he's the only one who knows about it.
Queen Tehedra doesn't get to cheat with whom she likes; her husband, the high king Jakoven, appoints her lover. He, on the other hand, cheats on her with as many young men as he can lure into his bed.
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.
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 Jacqueline while married to Linnet.
Five Little Pigs, Amyas Crale was constantly having affairs, but always returned to his wife in the end. The latter reason is why he was killed.
In The Murder at the Vicarage, Anne Protheroe and Lawrence Redding are having an affair (not really a spoiler. It's revealed comparatively early in the book.)
In The Mysterious Affair at Styles, John Cavendish is famously involved in an affair with Mrs Raikes. Similarly, his wife Mary gets far too comfortable with Dr Bauerstein.
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, and 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 Charles 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.
All over the place in The Chronicles of Magravandias. Pharinet and Valraven carry on an affair while married to the Leckerys, Varencienne sleeps with Merlan later Shan and Khaster, with the implication that one of them fathered her third child, Khaster takes up with Tayven while still married to Pharinet, Tatrini implies that she and Leonid both took lovers... basically a side effect of the arranged marriages.
During his marriage to Helen in The Tenant of Wildfell Hall, Arthur Huntingdon commits adultery twice that Helen and the reader are aware of—with Lady Lowborough and his son's governess. However, he makes it clear to Helen that he has fooled around with married women before.
Elizabeth Wakefield in Sweet Valley High. Actually pretty much everyone in the world of Sweet Valley although Elizabeth gets away with it the most.
Both Victor Henry and his wife Rhoda have affairs in Herman Wouk's The Winds Of War, he with Pamela Tudsbury and she with Palmer Kirby. Pearl Harbor intervenes, but in War And Remembrance the affairs resume, and the Henrys divorce, each subsequently marrying his/her lover. The new couples remain friendly toward each other.
Horatio Hornblower's eponymous protagonist, while otherwise a more decent individual than the era would normally allow, is not a faithful husband. First he almost steps out on his Unwanted Spouse Maria with Lady Barbara, whom he genuinely falls in love with; later he does have an affair with Marie Ladon while a fugitive in her father's house in France. These are somewhat sympathetic cases, since both times Hornblower was far removed from his wife and, unlike poor Maria, Barbara and Marie are intellectual equals. Then in Commodore, after he's married Lady Barbara, he has a (heavily implied) one-night stand with a Countess while drunk and somewhat bugged out over the fact that his translator almost murdered the Czar. And in Lord Hornblower, he starts thinking about Marie again even before getting the orders that put him on a mission to France. Neither Maria nor Barbara ever learn this (being way geographically distant when Hornblower does this).
The Great Gatsby. Tom is openly cheating on his wife Daisy with another married woman, Myrtle. Then he gets pissed off when his wife begins an affair of her own with Jay Gatsby...
Rose has done this multiple times, but only properly gets called out on it in Last Sacrifice by Adrian.
Mia Rinaldi slept with both Jesse Zeklos and Ralf Sarcozy while still dating Aaron.
Daniella Ivashkov is married to Nathan Ivashkov but sleeps with Ambrose.
Eric Dragomir had an affair with a Las Vegas dancer while seemingly happily married.
Prince Rufus Ivashkov is married. According to Adrian, the Prince maintains two secret mistresses.
City of Fallen Angels reveals that all wasn't fine and dandy for the Lightwoods after the Circle broke up. Robert blamed their wrecked lives on Maryse, and it seems they were on the verge of separation/divorce but decided to stay together for the sake of Alec and their unborn daughter. Unfortunately, a few years later, Maryse found out about his having an affair with somebody, and confided in Isabelle about it. Still, they stayed together, even if Robert was spending more and more time away.
Dragonvarld: When Melisande is rescued by a stranger from a non-evident threat, Bellona incorrectly concludes (with the villain's help) that her lover is eloping. Since the two are, respectively, high priestess and captain of the temple guard, the misunderstanding has major implications. The two fugitives do end up developing a degree of attraction, but their eventual sex is the result of a Love Potion, not choice. Neither of them knows about the potion, however, and both consider themselves to have betrayed their partners.
In The Shattered Kingdoms, Kira had a brief affair with her husband's brother, but any immediate repercussions were overtaken by her husband getting exiled to what was supposed to be his death. She then becomes the mistress of the Emperor, but this time, it's not hormones — she knows that her husband actually survived, and wants to be in a position manipulate the Emperor away from any course that might result in this being discovered.
The Magicians: Quentin cheats on Alice with Janet after having a long period in which he's clearly attracted to her. Alice finds out, having sex with Penny in retaliation. Their relationship quickly disintegrates over this.
The Return Of Conannote not written by Robert E. Howard begins a year after Conan the Barbarian married Zenobia, the slave girl who saved his life in The Hour of the Dragon. It is noted in the early chapter that "Those who know Conan best are a little bit surprised to see the King, in middle age, turn monogamous and even uxorious". At the Royal Ball of Aquilonia, courtiers can't help noticing that the King and Queen are obviously in love and can hardly keep their hands off each other. Then, Queen Zenobia is abducted by an evil magician and carried off all the way to China. Conan immediately sets off to save her, going all by himself all the way from Western Europe to East Asia, braving countless dangers and repeatedly risking his life countless times on the way. There could be no doubt of his love and devotion to Zenobia - still, while en route he is not precisely monogamous or uxorious. He spends an intensive night of love-making with Devi Yasmina of Vendhya, an old flame of his, and later accepts the sexual reward offered by a Chinese girl whose life he saved. He also shows a considerable interest in the beautiful Turanian spy Thanara, though she had shown herself a cunning and dangerous enemy. In the very end, when Conan and the freed Zenobia make their long way back, Thanara makes a final attempt on Conan's life, and is killed by an arrow from Zenobia's bow.