Here's an article on Ken Cuccinelli, Virginia gubernatorial candidate and VA Attorney General, who filed an amicus brief with the Supreme Court that "laid out an argument against the constitutionality of allowing same-sex couples to marry." The article says that Cuccinelli and Greg Zoeller, the authors of the brief, "used a novel justification to make their point in one section of the 55-page brief — namely that gay marriage could lead to polygamy."
Specifically, they wrote: "Responsible parenting is not a justification for same-sex-couple marriage, as distinguished from recognition of any other human relationships. It is instead a rationale for eliminating marriage as government recognition of a limited set of relationships. Once the natural limits that inhere in the relationship between a man and a woman can no longer sustain the definition of marriage, the conclusion that follows is that any grouping of adults would have an equal claim to marriage."
This may or may not be a "novel justification" in law, but it's hardly a new idea if you spend any time on the internet and read the foaming-at-the-mouth ravings of anti-gay commenters on any Yahoo article. Such commenters often rant angrily of the slippery slope effect. "If you let gays marry, then people will be able to marry their five-year-old children, or their dogs, or fifteen neighbors!" Since marriage has to involve consenting adults, there is no real danger of anyone being permitted to legal marrying small children or animals. But the "slippery slope" issue as it pertains to polygamy is a legitimate one, and in fact it raises the question, why not?
Polygamy is not an unusual or shocking idea. It's been practiced by many human cultures, although because men have traditionally had most of the wealth and power, it's almost invariably polygyny rather than polyandry. Furthermore, it's clear from how many people "cheat" in supposedly monogamous relationships that seeking multiple partners is not an unnatural tendency by any means. It's also a popular subject in erotica (you can find examples by searching on Amazon for "menage"), and there are people who engage in polyamorous relationships all over the United States. You can even make a very reasonable argument that it's a Biblically sanctioned form of marriage (not that atheists care, but many of the people foaming at the mouth should).
In short, there's nothing innately wrong with polygamy, though a lot of Americans have a knee-jerk opposition to it because the only polygamy they've ever heard about is radical Mormon men living in isolation with ten barely adolescent child brides. Clearly, men marrying children is wrong, whether they marry one or a dozen. But polygamy, a form of marriage in which men and women can love and marry freely-- what's wrong with that?
Apparently quite a lot, judging from the Wikipedia article on the subject. In practice, polygamy often seems to typically involve child marriage. An extensive quote from Wikipedia:
"Polygamy has been condemned as being a form of human rights abuse, with concerns arising over domestic abuse, forced marriage, and neglect. The vast majority of the world's countries, including virtually all of the world's developed nations, do not permit polygamy, and there have been growing calls for the abolition of polygamy in many developing countries...Many international human rights organisations as well as Women's rights groups in many countries have called for its abolition where it still lingers. The practice has also been explicitly ruled to be a violation of the internationally binding ICCPR, for polygamy violates human dignity and equality, and the United Nations has thusly recommended that the practice be abolished everywhere by sovereign states."
Once again, we seem to be defining polygamy as a way of subjugating and abusing women. Perhaps it's often been this way in practice (and so, I would add, has monogamous marriage, which often treats women as property rather than as people), but there's no reason it should be this way in the United States if legalized (at least no more than monogamous marriage, which also can lead to abuse and subjugation, alas). Voluntary polygamy among consenting adults seems to me no more likely to lead to abusive situations than voluntary monogamous relationships. Indeed, it might lead to less abuse-- abuse thrives on isolation, and it's likely easier for an abuser to dominate and subjugate one spouse than five or six of them. Of course, this wouldn't be true of a single man with numerous women from a church that tells them they must submit to their husband's will. I will admit I'm thinking here of true polygamy-- several men and several women in a group-- without the taint of fundamentalist religion involved. To my knowledge this has never been tried on a large scale before.
Unlike gay marriage, however, there are some practical issues with legalizing polygamy. Our society is set up on the assumption that every person can only have one spouse (at a time, anyway!). If polygamy were allowed, the tax system would have to change, and so would health insurance (you could hardly expect insurance companies to cover ten spouses for the price of one). Divorce and wills would become far more complex-- dividing up property amongst ten people, and determining custody of children, is not as easy as splitting it up between two. These are difficult, but not insuperable problems. And they are very probably problems we will have to deal with, sooner or later. If several consenting adults want to marry each other, why are they less entitled to that right than any two consenting adults?