Actually throughout history polygamy has been the norm. Even the xian holy book has stories of men in polygamist marriages desopite xians claim that a traditional marriage is between one man and one woman. The fact is that the traditional marriage is an arrangement between one man and more than one woman. I have 2 theories on why monogamy became the norm. 1. Men who were in positions of political authority or men who were wealthy wanted a lot of women for their wives so laws were enacted that limited most men to one wife and the men of the political class could have more than one wife at a time. 2. Somehow women convinced men that they should be limited to one wife who they would be faithful to for the rest of their life. I think it stands to reason that it is unlikely that a man came up with the idea of monogamy for all men. It had to have been a woman who came up wit that idea. It just is not natural for a man to desire one woman for the rest of his life.