Misses the big point: Marriage in the United States is a powerful legal instrument for managing and coordinating civil, financial and legal contracts. The standards for "partnership" between two people have been well-defined and well-explored by 200 years of history. Extending those standards to people regardless of their gender is actually easier than discriminating based on gender.

Polygamy adds a layer of magnitude to the issue. Whereas before you've got two people and if they agree they're good, they don't they're divorced, adding one or more partners to the mix complicates the fuck out of things:

A) Jack and Jill are married. Jack is in an accident that puts him in a coma from which he may not recover. It is Jill's choice whether to sign the DNR papers and determine how to handle their estate ("the Hill").

B) Jack and Bill are married. Jack is in an accident that puts him in a coma from which he may not recover. Under 1man1woman, Bill is fucked. He may not even be able to visit Jack at the hospital. He has no legal right to settle Jack's affairs and is likely ineligible to remain on the Hill unless careful paperwork has been notarized ahead of time. THIS is the argument for marriage equality - Jack and Bill are a family and its discriminatory to rule Bill a second-class citizen simply for having a dick.

C) Jack and Jill and Bill are married. Jack is in an accident that puts him in a coma from which he may not recover. Jill and Bill get to fight about the DNR. Any decisions that get made have to be made by both of them. Unless one of them is "first wife." But that's not equal! What if we add Will to the mix? Now we've got a 2:1 voting bloc on all decisions. How do we separate property? Assuming durable power of attorney has not been assigned, how do we assign it? Longevity of bond? But what if Jack married Will because his relationship with Bill was loveless due to irreconcilable differences?

Who the fuck wants to deal with that shit?

Add in the hierarchy inherent in many polygamous relationships and the whole thing starts looking entirely too complicated for words. There's a reason polygamy died off in Europe but remained in Islam - title and land needs to be simple to follow and Europe was all about title and land. Islam, on the other hand, cared more about clan than geography and as a result, the social hierarchy was simpler. The more complex the holdings, the simpler the civil union. The simpler the holdings, the more complex the civil union.

Ask a divorce attorney how much fun he'd have dealing with a polygamous divorce. Watch his eyes light up. And then know, in your heart of hearts, "Why not polygamy."