Love Forgives All But Lust
A significant who accepts all of her mate's wrongdoings until he cheats on her.
Bob is a villain, but his wife/girlfriend Alice (it's pretty much Always Female) refuses to believe any accusation of wrongdoing on Bob's part, and prefer to believe in a vast conspiracy designed to discredit him, even if she comes across Bob beating a man to death with a shovel. However, this changes if/when Bob cheats on her, or flirts with another woman (or even talks to another woman), as she instantly becomes his worst enemy, willingly telling the heroes every last crime he committed and sparing no expense to see him hang. See also Arson, Murder, and Jaywalking when the other offenses make the cheating laughable in comparison, Woman Scorned for revenge in general, and All Girls Want Bad Boys.
- Sherlock Holmes:
- In The Adventure of the Illustrious Client, Holmes must find a way to prevent a marriage with a rich young lady and a depraved baron, who has already convinced her that a man of his quality has many enemies, who are happy to spread rumors about his philandering and having killed his first wife. When one of his victims fails to convince her, Holmes looks for a diary in which he counts his conquests (his "lust-diary", as Holmes calls it). Once Holmes delivers it to her (and the baron is disfigured by said former mistress), the marriage is called off.
- In The Hound of the Baskervilles, Stapleton's wife, who he passes for his sister, was his reluctant partner in crime until she discovered he was making advances towards another woman (whether or not he actually intended to propose is not known), which finally caused her to snap, forcing him to tie her up before she can warn the heroes. Too bad for him she'd already gone out to remove the pegs that marked a safe route through the moor.
- One Exbrayat novel features the murderer's wife caring less about the fact that her husband killed three people and more about the fact that he had a mistress.
- Taken even further in one Hercule Poirot short story: the man is (separately) romancing both a young woman and her aunt. He's trying to get money from the old lady and claiming that to prevent people from looking down on them both, he'll pretend to be in love with her niece. So when the aunt and niece have a fight (neither suspecting the man), the aunt sends Poirot a letter asking him to investigate.
- An interesting variation in the novel Rebecca by Daphne du Maurier. The female lead spends the first half of the book moping because she thinks her husband is still in love with his (dead) first wife instead of her. Cue wangst. But then it's revealed that he hated his first wife, and he actually murdered her. Murder? No problem! He doesn't love that minx; he loves me! (To be fair, it's presented like his first wife was The Vamp with absolutely no moral code and masterfully provoked him to do it... but still.)
- Family Guy: Peter develops amnesia and doesn't remember his family, including his wife Lois. She's relatively OK with that until he brings home his new girlfriend.
Hello, Unknown Troper. You'll need to get known to lend a hand here.