I'm not so sure about that. Sure the monogamy myth is cultivated in our culture, but that only explains a part of it. What about plain old jelousy? I don't believe their are people who aren't jelous at all. Some are just better at dealing with it than others.
Off-course there's always sibling rivalry which always boils down to jelousy. Getting more or less than siblings was when I was little a constant on-going battle. I'm certain that none of us always felt as equaly loved as the other siblings. Humanity is a greedy bunch, and denying it is just sweeping the problem under the carpet.
True true, but that's also the problem. I've never seen it happen. I have on TV, but that doesn't really count.