Starving_Artist
The science of human biology begs to differ. Humans are not monogamous creatures and were never meant to be. Male brains are wired to spread the seed and female brains are hard wired to seek out the Alpha Male who would ensure protection and superior genetics for their offspring. This doesn't mean that we don't feel love or can't be monogamous, it just means that we weren't exactly programmed for such a restrictive sexual code. Our species actually adapts and evolves better with multiple partners. A child receives the best portion of his mother and the best genetic portion of his father, as well as those of his grandfathers and their grandfathers. By mating with multiple partners we ensure that offspring have a better chance of survival because they're getting all of the best. Monogamy ensures that each child is no better or worse than the one before it and slows the pace of human evolution.
Oversimplification.
That doesn't take into account what sort of SOCIAL benefits come from monogamy. That we have biologically successful (meaning reproductively successful) societies within which monogamy is the predominant practice suggests that we are in some ways reaping major benefits from the arrangement.
One of the drawbacks that I can see coming from polygamy is in the investment of resources. Certainly, you could, as a male decide to just procreate like crazy to beat the odds regarding who survives, or alternatively, you could decide to instead focus your resources on raising a few children and raise the chances that each child has to "succeed" by limiting the spread of resources. With polygamy, a male's resources are spread out among his many mates...if a male decides, instead to choose a single "quality" mate, his resources are less distributed and he gets "quality" offspring. Considering that survival is no longer as much of a concern within societies like the U.S., it actually makes more sense to go with the alternative route.
And this doesn't even take into account that some females of our species (particularly within monogamous societies) get a big say to whom they get hitched.