You present a false distinction. I oppose same-sex 'marriage' not because it's somehow harmful to society, but because it's incoherent. The idea of "same-sex marriage" is a logical contradiction, something like a square circle. The idea that government can decide how to (re)define marriage is like suggesting that Congress can pass a law (or worse, the Supreme Court can decide) that tomatoes aren't actually a kind of fruit, but really they should be classified as a species of dolphin. If it's an empirical matter (which it certainly is), we're asking the wrong authority to comment on it; we should, as you say, be going to the great moral thinkers, not the government.