This's something I've been wanting for some time. A boy sees a girl he knows "with" someone else, maybe in a relationship, maybe being blackmailed or raped. Whatever the reason why the girl is having sex with this other guy or group of guys, by the end of the story the other guy(s) is gone and the girl is now doing it with the main character. If it approaches a sort of NTR-takeback (as in, the MC is in a relationship with her, or likes her, or thinks he's in a relationship with her) that's even better, though not entirely necessary.