@semck
On your first paragraph, I'd tend to agree. But I'd make a different conclusion than you, I'd say to take government out of marriage altogether. But it's already in it, and I doubt it'll leave, so why not make it legal for different actions? I still don't see an objective reason why same-sex marriage is less valuable than, uh, not-same-sex marriage. And no, neither religion nor status quo or good arguments.
- "The question, though, arises -- should the state actively encourage this behavior?"
Whoa there, two important points: point 1: legalizing doesn't mean encouraging. I'm all for the legalization of marijuana (and basically all victimless crimes), but that doesn't mean I want to give everyone free weed, or have the eagle in the Great Seal of the US be smoking a blunt.
Point 2: why is it such a bad behaviour? You're putting gay marriage at the same level as one would put gambling, alcoholism, or watching reality TV. Law needs to be based on objective standards and logic, which the prime directive being to protect the liberty of the citizens. If you're writing something off because you don't like it, or because it's banned in your religion, that's not democracy, or using logic, or protecting liberty, that's either an autocracy or theocracy, depending on which rationale you have.
- "by granting it formal recognition and attendant civil rights?"
This is the same recognition me or you would have if we wed someone of the same sex. What if someone deemed that marriage of any sort was "immoral," and banned you and I from having it? What if someone deemed that your religion, or ideology was "immoral," and banned it? This is the same rights we already have, and boy, I'm befuddled to see so many people let their personal beliefs get in the way and be in favour of restricting one person's rights because of something objectively trivial.
- "If it chooses to, then at that point it should have some reasoning to the effect that the arrangement is a positive social good that it wants to encourage."
Neither playing Diplomacy nor discussing things on a site made for playing Diplomacy have any objective "positive social good." Should the government ban it?
- "There, I'd need to hear some arguments."
It neither breaks your leg nor picks your pocket. Q.E.D.