Charles Krauthammer (linked here) takes on polygamy and same-sex marriage today and makes a lot of sense. Toward the end of the article, Krauthammer articulates the perfect blend of a libertarian world view and pragmatic logic:

"I'm not one of those who see gay marriage or polygamy as a threat to or assault on traditional marriage. The assault came from within. Marriage has needed no help in managing its own long slow suicide, thank you. Astronomical rates of divorce and of single parenthood (the deliberate creation of fatherless families) existed before there was a single gay marriage or any talk of sanctioning polygamy. The minting of these new forms of marriage is a symptom of our culture's contemporary radical individualism -- as is the decline of traditional marriage -- and not its cause.

As for gay marriage, I've come to a studied ambivalence. I think it a mistake for society to make this ultimate declaration of indifference between gay and straight life, if only for reasons of pedagogy. On the other hand, I have enough gay friends and feel the pain of their inability to have the same level of social approbation and confirmation of their relationship with a loved one that I'm not about to go to anyone's barricade to deny them that. It is critical, however, that any such fundamental change in the very definition of marriage be enacted democratically and not (as in the disastrous case of abortion) by judicial fiat.
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I agree completely, but I would tweak one minor point in the essay: Krauthammer points out that he has predicted for ten years that "it is utterly logical for polygamy rights to follow gay rights."

I would argue that polygamy would have made more sense as a trailblazer (or Trojan Horse) for same-sex marriage. By that I mean, polygamy is eminently more defensible than gay marriage. Gay marriage draws the ire of Natural Law theorists and scripture-emphasizing Christians. Polygamy, on the other hand, does not offend Natural Law devotees and has deep Biblical roots. For the most part, polygamy has only American tradition to overcome, which is not an insurmountable hill to climb.