Re: Or the opposite could happen....
Rubbish. Human society is no longer purely driven by "survival of the fittest" in its most basic sense. Otherwise e.g. there would be no ugly people, no one would marry or care for a disabled person, etc. We have rules that we have deemed make us more advanced than that. Yes, a male monkey will shag his way through all the females in the group, but humans, in most societies, have very strongly ingrained expectations of monogamy and fidelity. It's not just rules, it's deeply rooted social mores. These have been universally agreed upon because they make better the lives of individuals, and make society work better. It doesn't apply in all societies, and often the taboo of cheating is broken, for we are fairly pathetic meatbags with barely any self control - but to return to the other poster's point, cheating absolutely is a very harmful act. Just wait until it happens to you, and you're howling your guts out...