Just like Southern states couldn't be trusted to create schools that are separate but still truly equal. Hypothetically, it would work. But in practice, it depends far too much on the continued support of people who would rather we didn't have "garriage," or whatever we would call it. It wouldn't be long before someone would introduce a law that benefits married couples but makes no mentioned of garried couples.

Whereas, if SCOTUS forces states to allow gay marriage, y'all could continue to call it whatever you want, while we could be secure in the knowledge that, as far as laws are concerned, it really is no different from marriage and no future laws could forget that.