Should be an easy enough question to answer. Our sexual physiology seems to put us squarely in the middle between the biological expectation that a male will have to compete with other males scrupin’ the mate and the male having exclusive access to the mate (or mates). The size of the testes, number of sperm, layout of anatomy wrt impregnation, behavior patterns…all of that seems to hint that humans are in between related species that have either open mating where a male would have to assume that his sperm is in direct competition with the sperm of another male that might have already beaten him to the punch, and species that have nearly exclusive access to their mate or mates. Say, bonobo vs gorilla, with humans somewhere in between.
So, assuming I’m not totally off the deep end here, the basic building blocks of our sexuality have been in place probably from before we even were our current species. Adaption and change to get us where we are today takes time, so we are talking 100’s of thousands of years…probably a lot more.
Then there are the social aspects. Even more a WAG than the above, I’d guess that once humans started to communicate and to form social and familial groups that some sort of social compact or rule that emphasized something like monogamy entered into society purely to mitigate sexual strife as well as it could be mitigated. Being humans, and we continued to adapt mechanisms to deal with both monogamy and, um, non-monogamy using biological as well as social mechanisms, until we have the hodge-podge of today’s tangles of sexual norms, mores, customs and rules.
I don’t think an exact date for when religion first appeared in human beings (my guess is it’s a lot older than people think, at least if you define spirit worship, animal totem worship and the worship of nature and natural events), but think our sexuality was formed long before humans developed religion…probably several species before we became human. Rather than religion being the driving force for monogamy, my WAG is that it was the tight knit hunter gatherer societies who needed full cooperation and respect, and a minimum of strife to ensure survival that first brought the social aspects of monogamy to humans (or even some of our pre-human ancestors). While it’s possible to have a tight knit society without monogamy (there are primitive peoples who use other forms of bonds), I think it’s one of the easier ones to set up given the hardships our ancestors went through trudging out of Africa or facing the various ice ages.
Anyway, those are my WAGs, FWIW. Monogamy (and all of our other sexual baggage) pre-dates religion, and religion was secondary, at best, to the formation of sexual bonding in early human and pre-human society. Later on, religion was able to take the basic building blocks of human sexuality and form and shape it into a more formal event, but those things came much later IMHO.
(And before anyone asks, the above is my take on it, based on things I’ve read and stuff I learned in school, plus a healthy or unhealthy dose of History Channel, Science Channel, Discovery, NATGEO, NOVA, TLC and all the rest, so large grains of salt are probably in order)
-XT