For a poly relationship, cheating would be a deal breaker. For FWB-type relationship, cheating is a deal breaker IF I respect the relationship/other person.
I'm a little morally ambiguous on the subject of cheating, I guess. I've posted elsewhere that I don't always see it as a bad thing- it depends on how one deals with it after the fact. And I certainly don't see it as the devastating, crushing thing that it seems to be to some people (but that's just as it applies to me). I'd have a WAY bigger problem with MC, like, gambling all of our money away than sleeping with someone else without telling me first (which is our agreement).
I don't respect all marriages just because they're married. I would encourage a FWB to stop lying and fix or end their relationship, and would probably remove myself from the situation if my presence was making things WORSE, but there's a bit of "none of my business" in there too.
Pan Female, Hinge in a V between my mono (straight) husband, Monochrome and my poly (pan) partner, ThatGuyInBlack