Foe Cooties

"Joanna: How dare you judge me? I mean what are you? You think you're some kind of, like, angel here? No, you're just this penny-stealing... wanna-be criminal... man. Peter Gibbons: Yeah, well, that may be. But at least I never slept with Lumbergh."

- Office Space

Alice refuses to date Bob after learning that he once slept with Alice's enemy. It's usually portrayed as a visceral revulsion, whether or not it's justified by other factors such as infidelity.

Film

 * Gangs of New York. "I'm not interested in The Butcher's leavings."
 * Chopper. The eponymous character suspects his hooker girlfriend of having had an affair with one of his enemies while he was in prison
 * Office Space. We're treated to a horrific Imagine Spot.
 * Could apply to Bud's reaction when he learns that his hooker girlfriend Lynn slept with his enemy Ed in L.A. Confidential. It's ambiguous whether he's mad at Lynn for "cheating" on him, or for "cheating" on him with Ed specifically.

Live-Action TV

 * Happens to Ted in How I Met Your Mother when he's dating his frenemy's ex.
 * Veronica Mars can barely look at Logan on learning he slept with Madison Sinclair, let alone date him.
 * Happens between Olivia and Peter in Fringe, with the added twist that the enemy is Olivia's own Alternate Universe double, and Peter thought at the time that she was Olivia.
 * Chris Tate in Emmerdale married his girlfriend despite knowing she had cheated on him with his sister. He was much less forgiving however when he discovered she was cheating on him again with a man he despised (who happened to be her cousin) and got his revenge by committing suicide, having just discovered he was terminally ill, and framing her for murder.
 * This trope is used several times in Buffy Verse. Angel never liked Buffy having new boyfriends but was particularly bothered by her sleeping with his long-time rival Spike. A flashback in the Angel series revealed that The Immortal, a nemesis of both Angelus and Spike, had sex with both their girlfriends at the same time.
 * In Gossip Girl, Blair breaks up with Chuck after only being back together a few hours when she finds out he slept with her enemy Jenny Humphrey right before their reconciliation. As in, minutes before their reconciliation. She's unable to forgive this for a long time.
 * Also, Chuck is understandably horrified when he finds out Blair slept with his Evil Uncle. And Blair's not thrilled that Chuck slept with her enemy Vanessa. And Chuck's disturbed when he finds out Blair slept with his enemy Carter. They do this a lot, but Jenny is the first one which is treated as really difficult to forgive because it was after their long relationship instead of before.
 * In The Sopranos, Tony was more than happy to have sex with his rival Ralfies girlfriend Valentina but refused to make her his permanent mistress until he found out she never had sex with Ralf (due to sexual tastes being for S + M rather than conventional sex).
 * In Once Upon a Time, Emma reacts like this to finding out that Graham is sleeping with Regina. He seems to be ashamed of it himself.

Web Comics

 * Invoked in Something*Positive, although he eventually gets over it.