I generally stay away from politics here because I think they are much too prevalent and polarizing in the modern world. Too often people feel that the other side is all stupid or evil, and everyone gets their news and commentary from sources that treat the other side unfairly, e.g. repeating every slip of the tongue to put the other side in the worst light.
Gay marriage is relevant here, though. My take is that governments should do only civil unions for everybody, gay or straight, exactly the same. Government has an interest in matters like the proper disposition of property when a member of a couple dies without a will, that legitimizes them defining civil unions.
They have no business defining marriage at all. For religious people, marriage is a religious sacrament, and in the USA at least the First Amendment says "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof;". If a Unitarian church is prevented from marrying a gay couple, (or a Baptist church were forced to), their free exercise is abridged. Marriage should be up to whoever the couple wants to solemnize their marriage in the eyes of their community, including if that's nobody but only each other.
I have no idea what the civil union rules should be for polygamy (how inheritance should go etc.), but I'm sure it'll be much easier to settle them logically if we can agree that their LDS church or Muslim mosque or whatever organization has ALREADY married them according to their own practices.