Theory: Monogamy exists to protect the weaker (physically and sexually) of our species.
Without monogamy as a societal standard, the "alpha males" and "alpha females" would have far greater power than they already do.
Once power structures formed that stopped guaranteeing that the "leader" of any given group will have possessed great "breeding prowess", those in power were no longer necessarily the physical "alpha male" or "alpha female." They were quite often the opposite in fact, and chosen only due to their own birthrights not their own prowess at anything really. Beyond that the "intellectuals" who didn't necessarily curry natural favor from the opposite sex also came more to power.
They rather arbitrarily defined monogamy as some moral standard, and rather arbitrarily labeled promiscuity as amoral. Something the "alpha" of a species would never really do.
This rather arbitrary importance put on monogamy and lack of promiscuity placates the feelings of jealousy and insecurity the "non alphas" feel, because it effectively removes large swaths of the breeding population from the grasps of the "alphas." It also in turn feeds these feelings of jealousy and insecurity by teaching people from the time they are born just how important it is to give yourself to a single partner... and teaches many would-be "alphas" to ignore the potential power their genetics have bestowed upon them.
Since men have largely held the most power, they've also put special onus on women not being promiscuous.. while doing their best to keep some sort of double-standard alive whereas men are admired for their promiscuity, and women are made to be ashamed of their promiscuity.. even to the point where they are put to death for not obeying the "rules" in place in some societies. Throughout history men with "power" have expected their own promiscuity to be ignored, expected, or simply put up with... while using their power and status to get as much sex as possible.. because that's what they are naturally wired to do in the first place. At the same time they continue to promote the idea that everyone else should be monogamous.. because hey.. why not promote the idea of the "rest of the guys" not being allowed to play the game like they do?
Catholics take it as far as basically worshiping one and only one woman.. who was born a virgin, and died a virgin.. the "perfect woman".. lacking the "sin" of having sex with someone she is attracted to.
What a mindfuck.
I believe this repression of natural sexuality has done little to actually further our society. I believe "monogamy as a standard" exists to placate the weak, and feeds on weak emotions like jealousy and insecurity.. it allows non-alpha men and women to have some sort of "control" over their mates, by giving them an arbitrary moral "rule" to chastise people with and discourage them from having said control.
Just a theory