Oh, boy.
I have always thought the "gay marriage" issue was a pointless thumb in the eye of religious conservatives. The rights themselves are mostly--if not entirely-- available already through civil unions. This isn't an argument over civil rights, but about the desire to use the same word as the religious people.
I understand that historically government has had a hand in marriage...because of adultery laws no one enforces any more. Marriage is now a religious thing, so it is a violation of the separation of church and state for the government to legislate any definition of it. Government should be getting out of this, not into it.
The real concern with opening up the definition of marriage is the subsequent breakdown of the dyad structure. If you conceptually divorce reproduction from marriage--as gay marriage does--then there's no reason to stick to marriage being between two people. And no, this is not slippery slope: this is one of the things the Supreme Court is worried about.
And the problem with polygamy is that it implies war and imperialism.
Multiple marriages has historically ALWAYS ended up with men having multiple wives, and almost never the reverse because women marry-up more often than men for biological reasons. Allow polygamy and there will be a shortage of eligible women, especially for the lower class. War is a handy and logical way to kill off the excess lower-class male population.
Gay rights is a good issue to discuss, but gay marriage has some very unpleasant implications. Implications I don't think most people have thought through. I don't think giving 5% of the population inheritance and hospital visitation rights is worth risking locking us into war-economy imperialism. We need to find another way.