My question is -- why is this question geared towards men? Women cheat, too, and it's not okay for them to cheat, either.
I do think though, if the issue is that one partner wants more sex than the other partner, they should try discussing it first.
If they can't discuss it, or it can't be resolved, I guess they've got to see if this is a deal-breaker. Cheating should not be necessary at all. If you're not getting what you want in the relationship, move on.
As for polyamorous relationships -- I understand cheating to be seeing/sleeping with someone outside the bounds of the relationship. If the rules of the relationship are that both of you are allowed to sleep with other people if the partner approves, then you're only breaking the rules if you sleep with someone without your partner's approval.
And this became longer than I intended. For that I apologize, and I hope I came across as clearly as I intended.