The Alphabet Story uses this trope in several ways. Elsa and Kyra begin a Secret Relationship because same-gender romances are illegal in their countries. Kyra begins dating a man to keep up appearances, which means she's cheating on Elsa but also that she's cheating on Ludvig with Elsa. Kyra and Elsa break up because they dislike cheating behind Ludvig's back, however it's implied they'll end up back together.
In What About Witch Queen?, prince Eric of Weselton's wife Margaret cheats on him with The Spymaster Glenn Hunter, and she might cheat on him with smuggler king Felix Drachner. He suspects the latter but has no idea of the former.
The Light in the Dark: Bertha Jorkins' father cheated on Mrs. Jorkins with another woman, resulting in Bertha's birth and Mrs. Jorkins' hatred of her. Eileen Prince is having an affair, but who with remains unknown. Meanwhile, both of Maria Zabini's parents are cheating on each other, but neither of them realize it. This actually serves as something of a Freudian Excuse for Maria and her skewed perception regarding relationships, as she has known since she was a small child and was sworn to secrecy by both of them.
Many Dramione fics start off with Hermione discovering that Ron is cheating on her (usually with Lavender) in order to get that relationship out of the way. Slightly odd, considering that Hermione and Ron didn't actually date during the books.
In Angel of Death, Trent has an ongoing affair with Karen behind his wife Elli's back. When Karen gets pregnant, Trent decides to perform an abortion himself in order to save his reputation as a doctor.
Her Scarlet Letter starts when Angela gets pregnant with Chase's child. Both of them are married, though Angela's husband Jin is away. The affair and pregnancy causes a huge scandal when it's revealed.
Sehnsucht: Defied. The close friendship between Daemon Spade and Cervello Cynthia had hints of an emotional affair — it's confirmed that Cynthia was in love with them, and it's implied that Daemon was aware of those feelings and reciprocated to some extent. However, the latter was also mutually in love and in a relationship with Cynthia's adoptive sister, Elena. Whatever the two of them had together, they tried to nip it in the bud for her sake.
Persona EG has several characters cheat on someone at major points during the story:
Cadance has been known to cheat on Shining Armor a lot, even before the events in the story itself began. However, the worst time out of all of them was when Cadence's relationship troubles with Shining Armor came to a head and a drunk Cadance seduces and sleeps with Flash.
After Sunset moves into the dorm, Shining Armor eventually sleeps with her during a visit at Sweet Apple Acres where he says she looks just like a high school crush he once had. It turns out he was lying; she really resembled a movie character he lusted for.
Advice and Trust: As they opened up to each other and talked about their pasts in chapter 1, Asuka told Shinji her father did not mourn her later mother long, hinting he cheated on her with the nurse was supposed to take care of her mentally-ill mother.
A Crown of Stars: When Asuka's mother Kyoko was mentally ill, her husband cheated on her with her doctor in the next room where Asuka could hear them. Several characters bring it up and Asuka declared in chapter 33 she does not intend to forgive him:
Asuka: "Oh yes, my wonderful and supportive father, who ditched Mama before she was even gone, and me before she was two weeks in the ground. He can rot in Hell in the cell next to Shinji's father. They can swap 'cheating on your child's mother' tips. He can stay wherever the Hell he is now. He'll get all the care from me that he gave my mother and I."
Doing It Right This Time: Subverted. Ritsuko accused Gendo of cheating on her with her own mother. Gendo replied he and Naoko broke up a good while before he and Ritsuko started dating, and Naoko was aware of it. Ritsuko was not fully convinced but she gave him the benefit of the doubt.
After reading her girlfriend's diary, Mandy thought Jessika had cheated on her with Asuka. In truth Asuka did not even know Jessika had a crush on her, but Mandy called her a "fucking home-wrecker" anyways.
During his bachelor party Touji got a lap dance from a stripper. Later he felt guilty and he confessed Hikari what he had done. Hikari eventually forgave him, but he very nearly ruined his marriage before it even started.
After the above events, Hikari's best friend Asuka informed Shinji that any hypothetical adultery would be punished with electricity to a specific male organ.
In Ghosts of Evangelion, Kyoko resented her husband for cheating on her. Ironically it turned out she had an affair with a married man when she was a college student.
The One I Love Is: The last straw that drove Asuka to suicide was finding Shinji sleeping beside other girl. Although they had not done anything Asuka did not know that, and she thought that he had betrayed her (and in the first -and discarded- version of that chapter Shinji actually had sex with that girl).
In Denounce The Evils, it turns out that Jessie and Cassidy have bad blood because they're exes who had a messy break-up. Cassidy cheated on Jessie because she couldn't handle Jessie's attitude and abandonment issues.
"Heis'he Ri'nanovai": Morgaiah t'Thavrau discovers that she was the product of an adulterous affair between her mother Iliana and her boss, Merken tr'Vreenak. Morgan surmises the two broke off the affair to hide her parentage from tr'Vreenak's wife, as this could've caused a three-way blood feud.