Men quantity and women quality. Over time, neither produces monogamy.
While men want to be with a wide array of different women and different quality at the same time (meaning, from hot to not so hot women at the same time), women want to always be with the hotter, top and most solicited flavor of the moment.
Men want the most deals they can get. Women want the best deal they can get. Men want to open many doors. Women want to open the right door. Men are about taking risks and pushing the limit, losing big or winning big. Women are about doing the safe bet.
So women want the hot flavor of the moment, the same that everyone wants: the safe bet. And men are constantly fighting each other, often to death, to be on that hot top spot, because being on the hot spot is the key to open every door. Since the men in the spot are replaced often, women tastes also change often.
Disclaimer – women dont just want to “fuck” but they want “relationships” with the guys on top, and by relationships I mean they want complete dedication, attention, resources and focus from the guy they pick, and want that to last, at least, enough time to produce beautiful offspring. By “relationship” I dont mean they want to support the guy, be his partner and stay with him because he needs it. Its not a relationship for the guy, but a relationship for her. Cool?
So women want to breed and be associated with the flavor of the moment and the top guy, and will always want this, to be with one of the top guys – not with the top guy she picked a while ago, after he is no longer on top.
Monogamy? for the man, its unnatural. For the woman, only as long the guy she is with is the flavor of the moment – the king. Only that the king, in our culture, is so relative and brief.