[old and tired]

What prevents a couple from living together and calling it marriage?

Nothing. What prevents it now?

[ClearCase_guy]

Seems to me that if two men live together, they can just say to themselves "We're married" and the government has nothing to do with it. If three people live together and say "we're married" then there is nothing I can do about it.

But the government can declare that a man and woman can form a special union and this can be encouraged and recognized as valid and important.

I see no value in giving up and just saying "Whatever you like" to such a crucial institution. I'm a social conservative and proud of it. Government recognition of traditional marriage is a good thing.