As a species, we were never dependant on it. Culturally the constraints were placed upon us by people we don't even know thousands of years before we had to live by them. So, if we don't NEED monogamy, why are the overwhelming majority of us living that way?
Maybe the next evolutionary step for human beings is not physical at all. Maybe our next step is to accept, socially, that all lifestyles are equally valued in society.
I'm writing this in a hurry - so I apologize if it makes no sense at all.)