I'm having a bit of trouble parsing your comment. Are you saying that if Bob had sex with your mother, you'd be pissed off at Bob, because this would mean that your mother has cheated on your father with Bob...? Fair enough, I suppose, though it seems to me that Bob in this situation isn't the one who's broken any promises/agreements; in general the blame for cheating seems like it should be assigned to the cheater, not the person he/she is cheating with.

... but this thread is probably fast approaching an entirely too tangential state relative to the main post.