Actually if you are a great hero, leader, zillionaire etc. you still can't have multiple wives. You can take on multiple sexual partners which are known as concubines. You have a duty to raise the children and support children from concubine unions, but oftentimes they do not inherit a title or estate. This is because marriage has to be sanctioned by the community. You need approval of the state, community or whatever organization to get married and the other mate must also be "noble" in order to have children that inherit that title. If you are of the lower class I guess it doesn't matter.
But anyway the concumbines also have no real rights in the marriage. Their only purpose is to be a sexual partner and they raise the kids and the father supports them financially. The wife however serves a function in that she can speak on behalf of the husband, share in his wealth etc. A normal person could even have a concubine instead of a wife I guess. The main thing here is you can't have causal sex without some sort of union that is made to ensure the proper rearing of children. Normal men are pretty much limited to one unless there is some kind of war that wipes out all the men. Great men are able to spread their genes.
But there can't be two wives. Cultures that have 40 wives its because the women have no rights. They basically are concubines. In our culture sense wives have rights, duties etc. (such as if the man dies she inherits everything and rules his estate) there can't possibly be room for two such people. A concubine is almost like friends with benefits exept that she can't really run around with multiple men and the man has an obligation to take care of children from the union. Though the union can also be dissolved easily sort of like modern marriages. Divorce with a wife in traditional society is basically unheard of. That is because the union is out of duty to the community, children each other etc. It doesn't matter if the union no longer seems convienient for you out of obligation to the social order you stay married. I guess if the two people really got sick of each other they could both seek out outside mates but that would be very rare and impractical.
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