One way to end democracy is for the majority become nihilistic and self absorbed, and attempt to lead the entire culture towards decline and death. When that happens, the minority need to treat the majority as though they were an occupying force, and fight for their lives.
The measuring stick for this is not really based on empiricism... it's based on the opinion of large masses of people. If the culture is really legitimately sick but a critical mass of people don't band together and care, they'll all just die off. If the culture is not legitimately sick but a critical mass of people believe that it is, that's still enough to create a civil war.
Personally, I agree with Orson Scott Card. I think the right way to view marriage is as a bargain between a man and a woman, and their community. The couple gets support, and the community gets another generation of mankind. I don't think there's any utility in actively persecuting people for their sexual orientation, but marriage is a mechanism for the community to fund the creation of new life, not just two people making a statement of love for each other.
In order to really hammer home that this is not about prejudice, and refocus the marriage laws around the common good, which is what they should be about, I think the right course is to institute mandatory annulment of fruitless marriages after a period of time... 3 years sounds reasonable.
If you marry and collect the financial incentives, then get an annulment, you should only get one "free pass"... if you then remarry and get a second annulment, or if you get a divorce, you should be forced to repay the money.
When gay people point at these train wreck celebrities who have 6 fucked up marriages in a row and put up stupid slogans, they have a real point. When they point at marriages that don't have families and children, but just a man and a woman getting a discount on their taxes because they promised to share a bed, and say they are no different, they have a real point.
The correct response, I think, is to address these points and make marriage about funding new life. That is, after all, it's social purpose... the human need it meets that justifies its existence. Really, unless gay people look forward to living their retirement years in a culture that has no young to keep the lights on, they should DESIRE a mechanism to pay other people to take responsibility for bearing and raising children... they should not be fighting to subvert it.