Here is a post I made in a comment about In Defense of Polygamy. The comments (some of them) are as interesting as the entry; if you want to comment, you might comment there.
It is important to distinguish polygyny so far as (male) resources are concerned and polygyny so far as sex is concerned. A better approach than a society of polygamy or strict monogamy would be one of what one might call monogamous polygyny, i.e., one in which males are able to care for essentially just one mate, but are able to have sex with many females. I am inclined to think marriage for the male should be about promising caring, but not about promising sexual exclusivity. It doesn't really hurt women for a male to have (meaningful, procreative) sex with multiple (clean) females so long as he didn't take much effort getting them, and so it is rather selfish of wives or would-be wives to claim they much want such exclusivity (most of the women claiming to want such probably don't really in fact want that, they just find it convenient to pretend males desiring responsibility-free sex are immoral inasmuch as creating such a belief in a would-be mate makes it less likely he would merely screw her if that is the extent of his affection).
There seems confusion present as to the disadvantage of polygamy. The problem is not that polygamy, by encouraging bad males to spread their resources about broadly like Halloween candy to trick-or-treaters, does punish highly talented females. No, that would be a good thing, because the mates of bad males are likely bad. What encourages talent to evolve slowly in bad women is good; not everything that is immoral is reprehensible (worthy of punishment or severe stigma). No, the problem is that polygamy doesn't sufficiently allow good women to be unselfish. In a Western Society, a woman has a real opportunity to choose quality vs. quantity. Bad women, wanting quantity, are more-or-less completely demanding of marriage, because marriage is quite the benefit. A good woman, though naturally she would prefer marriage, is going to be much more willing to merely (in the clean sense) fuck, if she finds a man she really loves but can't have in marriage. The practice of common-law marriages rather make this difficult, but traditionally (especially in France, where only marriage was considered legal proof of affiliation) and even now to a certain extent, western society has given to females the right to have children by males without the males being required to be responsible or to be afraid of being responsible. If marriage gives just a fraction of a would-be husband's resources, as in polygamous societies, marriage just isn't sufficiently selfish for women. And strict monogamy undesirably forces good women to mate males that they don't find beautiful or sexually pleasant.
Men not spreading their resources about like Halloween candy makes female selfishness significantly easier for females, which by encouraging bad females to be selfish, hurts bad males by causing talented bad males to not be able to get the especially rewarding responsibility-free sex as easily as good talented males can. And as is good, bad males can still be selfish (to their likely bad present or future mates) in a polygynous monogamy, because they have the opportunity to fritter away their youth trying to seduce females into responsibility-free sex, and because they can spend their money to "buy" a wife as opposed to being a good uncle or to taking risks for the greater good.
One disadvantage of polygynous monogamy is that when a male loves two women almost equally who are both willing to get fucked by him, it seems kind of arbitrary (and introducing of an undesirable noise in how sexual selection operates) that he should have to give all his resources to just one if he loves them. A way around this that would work in many circumstances would be that if a male can fuck a close friend or relative of some other female whom he loves about the same or just a little more, then he should be able to spread his caring about more broadly. In such a case, where a friend or close relative of some other female is willing to give sex for totally free, it is not likely that the other female is selfish when it comes to demanding his resources, and so strict marriage in this case doesn't particularly encourage bad females to be selfish.
As for non-strict monogamy encouraging loss of diversity, that is true. That said, it is not decreased diversity that is particularly bad or dangerous, but a high rate of decrease. And it is important that natural selection be balanced with sexual selection. Income differences make for natural selection (at least to the extent the income differences are reasonable). In a strictly monogamous society, unselfishness in sexual selection, which is mostly what drives the evolution of unselfishness, depends on incomes differences. The only significant way to mate unselfishly in a strict monogamy is to mate worthy poor people; the unselfishness depends on income differences. In an asymmetric (i.e., women take care of all their children, men only take care of wife's children) polygynous monogamy, there is ample opportunity for unselfish love in mating even if the society has small income differences. Income differences are necessary just so stuff gets done and so females love useful traits rather than useless traits like who is the most addicting in his depravities or who can mix his drinks or push his drugs most seductively. Anyway, it is very difficult to justify strict monogamy in an ideal society if such a society would (as seems evident) have less income differences. Of course, to be single mothers, females need the opportunity to become economically self-sufficient, something modern western society has more than other societies, but not as much as it should.