In the wake of federal Judge Vaughn Walker's ruling that overturned California's Proposition 8, that defined marriage solely as the union between a man and a woman, I think it would be useful to ask why not legalize polygamy (or polyandry, for that matter).
Understand: I am not advocating this. But if the US Supreme Court upholds Walker's ruling, as many observers think it will by 5-4 vote (Kennedy being the swing), the the inevitable - indeed, unavoidable - consequence will be that the state has no authority to regulate marriage. That being the case, it will not be possible to offer a legally sustainable argument that gay marriage is okay but polygamy is not.
Polygamy has in fact has vocal though few advocates in the United States for years. The Mormons began practicing polygamy shortly after moving to Utah in the 1840s, continuing with ecclesial endorsement until 1890, when church president Wilford Woodruff ended the practice its sanction. But the practice continued until for new few years afterward. (Presumably, existing polygamous marriages were grandfathered.) Some Mormon advocate re-sanctioning polygamy today (and here).
A post at Blogcritics on "Legalizing Polygamy and Polyandry" states the obvious: "that polygamists and polyandrists see the recognition of same-sex marriage as being a justification for their cause... "
I personally do not foresee a large movement toward polygamy (or "plural marriage," as its advocates call it). But my question remains: once the Court has struck down the states' authority to regulate marriage, what is there to stop it? Nothing.
I'll give the last word to the Chinese:
Above is the ideogram for "two women under one roof," which means "trouble" in Chinese. This may be an urban legend, though. "Yin & Yáng and the I Ching" says that no such symbol actually is used in China, although there are authentic ideograms for one woman under a roof and three under a roof, thus:
Interesting that three women under one roof has such purely negative connotations. And probably informative, too.