For example:
Ciguatera -
Spoiler (highlight to view)
When the protagonist sleeps with another girl, and his gf sleeps with her employer.
Welcome to the NHK -
Spoiler (highlight to view)
While the affair never happened, it was desired by the protagonist's high school friend.
Me-Teru no Kimochi -
Spoiler (highlight to view)
Incest in the form of a stepmother/stepson relationship.
Oyasumi Punpun -
Spoiler (highlight to view)
When Punpun's uncle's gf initiates intercourse with Punpun.
Would prefer if it was seinen manga dealing with cheating and/or incest. No hentai, yaoi, shounen ai, shoujo, or shoujo-ai. Thanks!