This is what you get for voluntarily involving the government in your personal matters.
What bugs me most about the entire gay marriage debate is that it implies that the government should have control over the institution of marriage. The solution to the gay marriage debate is not to make it legal for gay people to get married, but to make it so that the government doesn't decide who can get married.
For example, immigration sponsorship. As a gay man with a non-American partner, this comes to mind first.
Currently, spouses do not have to testify against each other in court. This isn't codified in any law but British Common law (afaik). There are a few of these pleasantries that would have to be figured out.
Then, there are over 1,000 legal rights codified into contract law. Some of these can be emulated (power of attorney), some cannot.
When Canada was still arguing about how to handle gay marriage, several Canadian politicians (both gay and straight) did, in fact, argue in favour of abolishing marriage as a legal institution. It was seen by many people at the time as a compromise position.
I'm not implying anything about the achievability of the abolition of marriage, I'm just speaking about it from a theoretical point of view. I completely understand that the social conservative movement is too strong in America for anything like this to happen in the forseeable future.