It's because one guy would whore all the chicks! 
take away the religious crap.
and marriage is really just nothing but a business contract. making certain paperwork official regarding money, kin inheritance, and other assets.
making certain tax things official with the government.
its all just a money game.
and the more money you have, the more woman you can handle.
its a very classic way of life in many cultures. works well to.
even with added human rights that other cultures lack, it would be a sweet deal.
obviously the poor, uneducated, unimportant people in society arent married.
so you see how awesome this is.