I believe monogamy becoming the norm is due to wanting to know who's whose child. There didn't use to be a way to tell who was the father otherwise, and when property started existing (in the form of land or whatever) and needing to be passed along, men would have wanted to make sure they passed it to their own children and not someone else's.
I believe that's the reason why monogamy was generally expected of women but not so much of males.
Then with gender equality came the notion that both should. Often gender equality comes in making the gender that has more freedom join the other rather than the opposite (or a compromise). It's rare that the gender who had less rights suddenly gets them all.
I think now we need to work on making it socially acceptable to decide for yourself what works for you without having to follow such models if they don't work for you.