The problem with polygamy in a peaceful and prosperous society is there will be huge inequality between the most attractive and the least attractive in terms of the ability to find sexual partners, which will be a strong factor in social unrest. Potentially there is a correlation between countries permitting polygamy and terrorism. Monogamy and polygamy are two sides of the same coin. I think over time society will choose the one best suited to its current circumstance, but it can take a while.