I recall one of the Evolutionary Psychology books I read discussing this (I think it was The Moral Animal). It claimed that polygamy was relatively beneficial to high quality males and low quality females; high quality males would end up with more mates and low quality females would end up with a higher quality mate than they would otherwise. For the same reasons, monogamy was relatively beneficial to low quality men and high quality females; low quality men would have a higher chance of finding a mate at all and high quality females would end up with a higher quality mate.