With all of the discussion over whether gay marriage will lead to legalized polygamy, there hasn't been nearly enough speculation on what the consequences of legalized polygamy would be.
The general concern about polygamy is that as it will likely be mostly polygynous (i.e. men having many wives), there will be a large number of men left without wives if it becomes widespread.
Historically, such circumstances lead to opportunistic homosexuality and an unhealthy level of competition between men (as in men will not cooperate with each other).
Would that happen in our society? I think not. Rather, I think that to the extent that polygyny becomes common, polyandry (women having many husbands) and group marriages will also occur. For simplicity's sake, I am going to ignore bisexual polygamy (i.e. one person married to people of both sexes), so that I can consider all polygamous marriages to fall into the polygyny/polyandry categories (group marriages can be looked at member-by-member and thus reduced to multiple examples of polygyny and polyandry). Because it is irrelevant to my analysis, I will also ignore issue of how different group marriages operate (e.g. is every man married to every woman and vice-versa, or are some of the men married to only half of the women, et cetera).
So the big question here is why would polyandry occur in the US when it rarely has occurred in societies which accept polygamy?
Here are my answers:
(1) Our society is more tolerant of female sexual promiscuity than many others. Not that people don't still think of promiscuous women as slutty, but come on, people are fascinated by Paris Hilton. And it's not like someone is ready to stone her for anything she's done. Generally, most societies have prohibited polyandry even if they accepted polygyny. If polygamy became acceptable in the US, it would be unthinkable to restrict it to polygyny.
(2) People have fewer kids. Essentially, this means that women could regulate the number of children they have to coincide with the number of husbands they have. She could have one child with her first husband and then insist he had a vasectomy. At that point, he won't have more children no matter what, so he's not in competition with other men (except to the extent that he is in competition with them for the amount of attention his child gets). Men would be more willing to share a wife if they knew that they would have as many kids whether or not she had another husband; in contrast, if women were having twelve to twenty babies over their lifetime, a second husband would not increase her total number of children, and so would likely mean that each husband would father fewer.
(3) Women are considered to have a lot more rights vis-a-vis their husbands. Essentially, this means that for women who would rather stay home than work outside, a woman with multiple husbands could insist on a certain amount of support from each of them - which would also, by the way, be a substitute for finding a rich husband. This would potentially mean that a poor women who wishes to stay at home could marry four or five men of modest means and have each of them support her to a degree, instead of trying to find one successful man.
For women who are more career-oriented, the multiple husbands could split the childcare responsibilities; a four or five income household could more easily sacrifice one income than a two-income household.
The only potential drawback is that women with multiple husbands might have to have more sex than they would feel comfortable with, but if they find the advantages of multiple husbands alluring enough, the pharmaceutical industry could probably whip something up for them. Or maybe that problem will be solved by sexually-revved up women on steroids.
That is all.