Hi,
Perhaps I am completely off here, but here's another theory.
The institution of marriage was essentially established not for love/happiness/a-warm-fuzzy-feeling but rather to ensure the maintenance (rather passage) of one's property (from one generation to another). Lacking a legitimate heir, chaos would ensue.
This would be far older than the Catholic church, most likely back to the general end of nomadic/hunter-gatherer society and the beginning of agriculturalism and the agrarian society (I would guesstimate 10,000 years ago).
Provided the male to female ration is close to equal, monogamy would always [eventually] prevail because - historically polygamy has been associated with the upper echelons society (and specifically with the farmer-class rather than the artisanal-class), this would be due to being able to afford (in one way or another) a larger family size - which would also mean each offspring gets a smaller part of the original property. (Which works positively when there is too much land to be worked on by one farmer.)
In other societies, which would be weaker economically, polyandry would prevail - the amount of children one woman (with multiple husbands) could have (as opposed to one man with multiple wives) is very limited. So the collective property of family unit would most likely be better divided for each successive generation (possibly getting larger rather than smaller as with polygyny)
[By economy, I mean, arable land; but it would obviously differ from one place to another.]
At this point, in Western Society at least, we no longer have [need for, individual] arable land, furthermore most family units don't have any substantial inheritance and/or property(if at all, houses being mortgaged, those loans where you get cash for your house when you turn XYZ, and it is still yours so long as you are alive) has kind of make the passing of property from one generation to another irrelevant and practically non-existent for the middle classes.
I don't see this as a modern trend, although it has greatly accelerated in the last 100-odd years (with overpopulation etc), but the idea of property ownership and "arable land" essentially started dying out (in western civilisation) with the fall of the Roman Empire and the rise of Feudalism; following which I believe the concept was ingrained in society too long to be simply tossed away, and of course religious institutions had their part to play in keeping the status quo.
Society's rules, though some might not make perfect sense in the present moment, usually made sense when they were formed (consciously or otherwise).
In most ancient tribe societies, both men and women had several lovers and among our closest relatives --the chimpanzees--(to quote Penn Jillette) "everybody f*cks everybody".
Not quiet, I believe there was a saying "for love there are boys and slaves, a wife is to bear children".
Marriage in those times had nothing to do with the concept of "love" - it was almost always a prearranged for economical reasons.
Arranged marriages have only fallen out of fashion for since about the Enlightenment.
Shiroi Tora: Commitment - when you are between a rock (social pressure) and a hard place ( the disgrace of divorce ) - how acceptable was divorce in even the 40s and 50s? What about bastard children? God forbid homosexuals!
People have no choice but to make the best of what they had, so they did. All these fairy tales we hear about lover eternal, till death do us part etc... It has a lot more to do with society than love, far more than one would like to think.
Just my 2c. I could be totally off on all points, or maybe I can't quite get my point across because I'm a terrible writer.
