The difference is, humans evolved a highly intelligent brain that allowed us to recognize and overcome our base instincts and organize societies that defined marriage in a certain way by law and custom. These millennia of collective human wisdom have been unraveled in the last 50 years or so and we are reaping the consequences today.

I like Steven Pinker's take on polygyny from How The Mind Works:

Legal monogamy historically has been an agreement between more and less powerful men, not between men and women. Its aim is not so much to exploit the customers in the romance industry (women) as to minimize the costs of competition among the producers (men). Under polygyny, men vie for extraordinary Darwinian stakes - many wives versus none - and the competition is literally cutthroat. Many homicides and most tribal wars are directly or indirectly about competition for women. Leaders have outlawed polygyny when they needed less powerful men as allies and when they needed their subjects to fight an enemy instead of fighting one another. Early Christianity appealed to poor men partly because the promise of monogamy kept them in the marriage game, and in societies since, egalitarianism and monogamy go together as naturally as despotism and polygyny.