It's not just the religious guys who say that "plurality of wives" is the "will of God." A lot of science guys like to say things like "the sexual dimorphism in humans indicates some propensity towards polygyny in our species." They may be right (the science guys that is, not the religious guys), but whenever I read of scientific fondness for polygyny, I feel like the science guys haven't really thought this through all the way.
Maybe I'm reading too much between the lines here, but I sometimes get this vibe of "Sorry, ladies, but that's just the way our species evolved..." while Professor Scientist is rubbing his nerdy hands together and secretly picturing himself surrounded by nubile babes.** It never seems to occur to Professor Scientist that in a polygynous society he might be one of the males below the fold: out-competed and out of luck. Or alternately that even for the top one-third of males who get all the babes to themselves, there's this little inconvenience of the the bottom two-thirds of males hanging around in the wings, waiting for the opportunity to kill the top guys and take their place...
What does the female armchair primatologist think of the polygamous tendencies of our species? Well, I'll tell you! Please keep in mind that all of this is opinion and speculation, not to be confused with actual scientific research.
For this discussion, I'll define polygyny as any situation where one male monopolizes multiple females throughout their prime reproductive years. So I'm not only including openly polygamous (polygynous) societies (like the FLDS), but also cases where a man has an official wife in addition to maintaining a mistress (or mistresses or concubines), as well as cases where a man takes a young "trophy wife" after his children by his first wife are grown. I'm not counting ordinary serial monogamy or egalitarian open/polyamorous relationships.
One of the most striking aspects of human reproduction is the tremendous parental investment in each offspring. Instead of each adult producing hundreds or thousands of young and letting the chips fall where they may, human parents devote a lifetime's worth of effort and resources to seeing just a handful of kids to adulthood. For humans, successful reproduction isn't just a question of quantity, but also a question of quality: making sure the kids grow up to be healthy and well-placed in society. So both male and female humans tend to seek a mate who will stay with the family for the long haul and provide a strong investment in their offspring. Investment involves not only an economic investment (food, clothing, shelter, etc.), but also social status and personally educating the children.
Despite the heavy emphasis on quality, however, quantity isn't irrelevant in human reproduction. An individual male can improve his reproductive success by attracting multiple mates as long as he can do it without significantly diminishing the quality of mates he gets and the parental investment he can expect from them. For example, in human societies whose upper strata involve some sort of nobility, old money, and/or established families, the mother's social/economic status matters quite a lot to the success of the offspring. A high-status male can't normally expect that a high-status female would be willing to share him (and his money/status) with another female on an equal basis. This often leads to the wife-and-mistress model: a high-status "official wife" is sometimes willing to overlook a side dalliance as long as the mistress (and her children) get little of the father's resources and inherit none of his status.
So what's in it for the mistress? Typically it's a case where any legitimate mate she could expect to attract would be low enough on the socio-economic ladder that she's better off with the leftovers from a fancier table. (Note that -- depending on the society -- this role in not always voluntary.) Since the father's personal attention is valuable, there has to be a very wide economic gulf between the alpha male and the omega males before the alpha's sloppy seconds start looking more attractive than the full-time attention of a lower-status father/mate.
The main thing that makes females tolerate and accept polygyny is lack of power and status of their own. It's my impression that polygyny tends to decrease as women are more empowered. In other words, when a woman can expect to command enough (economic) resources herself to raise her children to adulthood, getting a mate who will invest himself 100% in her and her offspring is a higher priority for her than finding a richer or higher-status mate that she might have to share. (The flip-side is that an openly polygynous society -- for its very existence -- essentially requires that women have no power or status at all.)
So in conclusion I'd say that -- even though some degree of polygyny is normal and typical for our species -- as we move forward to a modern, sustainable society, the amount of polygyny will (and should) decrease. (Take this opinion with a grain of salt, given my female bias.) Some bloggers have claimed that since "one rooster in the hen-house" is an efficient reproductive strategy for chickens, it should be for humans as well, but that view fails to take into account some of the realities of human reproduction. Firstly, in the chicken example, part of the efficiency rests on the assumption that the extra males can be dealt with by serving them up on the dinner table. And let's face it -- having a family to support makes a human male more inclined to try to be a productive member of society and less inclined to want to fight other males to the death (in wars and terrorism, etc.). Secondly, human children benefit from having more dad per kid. As I discussed in fertility, mortality, getting parents to focus more effort on fewer kids is our best recipe for a happy and stable future.
** This is not meant as a slam on horny male scientists. Anyone who follows my blog knows of my great fondness for them. As far as I'm concerned, mathematicians and scientists are the sexiest type of humans, followed by engineers. I know I ought to put looks first (out of some feminist sentiment of "turnabout is fair play"), but what can I say? I like what I like. ;^)
Besides, it's not like looks are irrelevant to me. Ideally a guy should be beautiful in addition to being brilliant. For example, take my husband, who's all that plus great at taking care of the kids and the house. Actually, don't take him -- he's taken. :D