In the 1500s, a pestering theologian instituted something called the Marriage Ordinance in Geneva, which made "state registration and church consecration" a dual requirement of matrimony.
We have yet to get over this mistake. But isn't it about time we freed marriage from the state?
Imagine if government had no interest in the definition of marriage. Individuals could commit to each other, head to the local priest or rabbi or shaman — or no one at all — and enter into contractual agreements, call their blissful union whatever they felt it should be called and go about the business of their lives.
I certainly don't believe that gay marriage will trigger societal instability or undermine traditional marriage — we already have that covered — but mostly I believe your private relationships are none of my business.