You obviously have approached this subject with a scholarly bent and seem to know quite a bit. I have thought a lot about it too and I have, from time to time, dabbled in Sociology and Social Psychology. I have a sort of negative, flip-side question for you: what do you think the origins and purposes of monogamy are? For the life of me, I cannot see how monogamy serves any real purpose except to guarantee mates for the less-advantaged males. What do you think?
Well, certainly your observation about monogamy serving to guarantee mates for less-advantaged males is accurate according to Psychology Today.
Most women benefit from polygyny, while most men benefit from monogamy.
When there is resource inequality among men—the case in every human society—most women benefit from polygyny: women can share a wealthy man. Under monogamy, they are stuck with marrying a poorer man.
The only exceptions are extremely desirable women. Under monogamy, they can monopolize the wealthiest men; under polygyny, they must share the men with other, less desirable women. However, the situation is exactly opposite for men. Monogamy guarantees that every man can find a wife. True, less desirable men can marry only less desirable women, but that's much better than not marrying anyone at all.
Men in monogamous societies imagine they would be better off under polygyny. What they don't realize is that, for most men who are not extremely desirable, polygyny means no wife at all, or, if they are lucky, a wife who is much less desirable than one they could get under monogamy.
Once upon a time when there were certainly no DNA tests to confirm paternity, monogamy was intended to guarantee blood lines so that wealth and property was kept in the family. How well that worked in reality is likely another story, but it did give (or was at least was intended to give) men control over who inherited property, since the right of women to own property is a much more recent development.
Monogamy's origins are also firmly rooted in Europe and the Catholic church. The church enforced monogamy as an aspect of virtue according to the gospel of St. Paul and other biblical sources who believed that Eve seduced Adam into sin and so women needed to be dominated and controlled to save them from sin. This also served to help men avoid feelings of jealousy by keeping their women to themselves, and women bought into it as well, perceiving that they preferred to keep their men to themselves, even as they were attracted to other men and tempted to stray - just as, in reality, were their men. The church declared that temptation to stray was a sin, and that monogamy was divinely mandated. Again, this was the church's way of controlling its parishoners and maintaining it's own power and authority.
From an evolutionary biological standpoint, humans are programmed to pair bond for the raising of children, though anthropologists believe we have never done so entirely monogamously. There is a reproductive advantage to spreading one's reproductive resources around to others, generally others perceived to be healthy - which explains why men in particular are so attracted to women younger than they. We women have a finite number of eggs and a finite number of years in which to put them to use - not so for men.
Next we throw into the mix the matter of sexual jealousy. This is not even remotely new in evolutionary terms and is believed to have developed as a further means of keeping parents together for the raising of children, especially in order to see that children are protected and provided for at least until they are out of the cradle. Some anthropologists assert that this explains why biologically limerence, that "in love" rush of emotions, eventually wears off, preparing us to then be attracted to others so as to continue to spread reproductive resources around to other attractive, i.e. reproductively viable, partners.
For more on all this, see the book references on the website for the Institute for 21st Century Relationships, specifically here and here.
I recommend the works of anthropologist Helen Fisher, especially her book "Anatomy of Love: A Natural History of Mating, Marriage, and Why We Stray", as well as the book "Christianity and Sexuality in the Early Modern World: Regulating Desire, Reforming Practice" by Wiesner-Hanks.
Sexual jealousy makes monogamy seem like a good idea, yet we are just as biologically programmed to stray outside the pair bond to further guarantee perpetuation of our species. In other words, it is normal to want to keep our partner to ourselves while we decline to remain monogamous ourselves. It seems that human evolution is entirely focused on outcomes and is not concerned with the internal conflict it may create in attaining them.
In my work with polyamorous people on resolving jealousy, I have encountered a few who are honest enough to fully admit that they want to be with others but are not willing to share their partner with others while admitting that it is unfair. A very few bisexual folks with straight partners take this approach as well, pointing out a need for one of each as justification for being with more than one while their straight partner has no such right or imperative. In both cases the motivation is to avoid feelings of jealousy while getting their need for variety met. These folks are definitely in the minority, at least as to their willingness to admit to these rationalizations. The vast majority of non-monogamous people share these feelings but are much more philosophically egalitarian. They choose to deal with concerns about jealousy more directly, by doing internal emotional work, communicating with their partner(s) about their feelings, and in so doing finding ways to minimize or resolve their feelings in a way that is fair to everyone.