I've wondered about that as well, since (1) women are multi-orgasmic and men not so much; (2) a man has enough of a challenge satisfying one woman, let alone multiple women; and (3) women tend to be more possessive and competitive, less wiling to share their men. I think it all has to do with going back to primal jungle days where it was all about where might made right, and man created initial societies where women (and the children they could bear) as property and sources of indentured labor. Unfortunately some societies haven't progressed much beyond that.