I'm looking for a type of plot that makes a very nice alteration of the typical NTR (netorare).
First, it flips the roles. Torare means something's taken from the character, tori means the character is the one who takes something/someone.
Second, it drops the "Ne", meaning that the other guy in some way doesn't really deserve to be the girl's lover. And not through some vague opinion about what he should have done or not done (with the assumption that he had a choice). The guy is somehow unworther for some reason, such as cruelty and deception, infidelity, or blatant stupidity on the level of actually making a girl do it with someone else.

I have three examples, two hentai and one manga: Oni Chichi, Rensa Byoutou, and "Ecchi Shite" by Ueda Yuh (12/10 issue of LO).