Are humans naturally polygamous ?Watch
Secondly, how do you determine, what is natural and what is socially constructed? Just because there are more polygamous societies - from this it does not follow, that this is natural. Maybe polygamy is a social construct and monogamy is truly natural, because monogamy is partly motivated by sexual jealousy and exclusivity, which is more in tune with human nature. Societies based on polygamy are also unfair and generally is reduced to - men get as many wives as they want, but women need to be loyal and exclusive to that one man. This is enforced through violence and coercion.