Did you read every other word I wrote, or something?
I'm not condoning cheating at all. Cheating is cheap and nasty, and it's a symptom for people who won't make a decision in their relationships. It's also people choosing $$ over marriage. Cheating is trying to have your cake and eat it, too.
If you don't like marriage, don't get married. If you don't want to be faithful, then have an open relationship. But loyalty, trust, and devotion existed long before there were marriages. They have nothing to do with rituals.
And like I said, people stay faithful because they consider how the other person would feel. And they love their partner so they don't want to hurt them. Hence they either broach the idea of an open relationship (so all the parties know the score) or they don't do it. The deception, they know, will hurt the other person.
And if they feel okay with hurting their partner, then they should end it, because I put it to you that they don't love their partner anymore.