Everyone forgets in any ethically non-monogamous relationship cheating can still happen. Usually this comes from communication breaking down via things like lying, deceit or inflexibility. That is a decision only you can make if that happened or not.
Ehhhh that's a question of definitions. Honestly I can't think of anything that either of my partners could do that I'd class as *cheating*. Might be lying or deceit, and that's a problem; could be any number of other things that would upset me. Cheating, though, is something different - it's secretly doing a thing that one is not allowed to do - and if there isn't anything that one isn't allowed to do, well, it's kind of hard to cheat.