Like sexually cheat on your partner?! In normal circumstances, I don't think there's ever be a situation in which cheating is considered OK by me.
Why I said "normal circumstances"?! Because this question reminded me of a scene from the TV series Spartacus. In that scene a Roman master ordered two slaves [a man and a woman] to have sex in front of him. The woman has a husband but she would probably be dead if she disobeyed. Now, is this considered cheating?!