I think men are more polygamous by nature than women are. Biologically and evolutionarily, it makes more sense for men to mate with as many women as they can, to spread their seed around and increase the chances of their genes surviving.

Women, on the other hand, as the ones who carry babies inside them for 9 months and then have to take care of and raise their helpless infants, thus we are more inclined to find a mate that sticks around to take care of his children, by providing for us while pregnant and while the baby is young. Less incentive for us to creep around and find another guy, as most men aren't too keen on caring for another man's children.

We're both hard-wired differently.

That's not to say that there aren't monogamous men and polygamous females, just that nature skews us more the other directions, I think.

"Marriage is a wonderful institution...but who wants to live in an institution?" -- Groucho Marx