We evolved to be really uptight about being cheated on because, for men, if you’re caring for someone else’s kids, you’re being screwed out of your evolutionary fitness. For women, if a guy is giving someone else some of his energy and attention and not you/your kids, then you’re not going to be as fit as you could be. There’s also a biological reason why we have an urge to cheat – guys are wired to increase their fitness by leaving as many progeny as possible, while women are wired to sleep with the most attractive men they can find so their kids can leave a ton of progeny, whether or not that guy is the best “provider” (who will be the one they’ll want to “marry” or whatever it was those crazy cro-magnon couples did.)
That’s why humans are socially “monogamous” but a lot of the time, we actually are not.
These deep genetic influences on our behavior have shaped our society’s views of morality, leading to the types of thing that @tinyfaery and @essieness were saying. Cool, right?
Anyway, at this point, what with birth control etc, we shouldn’t be as upset as we are when our partners aren’t monogamous. I know a few couples who aren’t (as an agreed-upon rule), and for some of them it works fine, but sometimes one partner gets caught up in a jealous rage, even when they’ve agreed to it and they themselves didn’t expect to. It’s a hard thing, trying to combat your biology. We evolved in a society that was a LOT different than the one we live in now, but our genes haven’t caught up yet. :)