The thing is, as an atheist, I do give a shit about marriage, and would like to get married someday. Not civil unioned - married. Yeah, it’s just a word, but it’s a word that means a lot to me. I’m kind of sentimental like that.
In the larger context of securing equality for gays, I think advocating this would be a disasterous tactic. The number one argument from the anti-marriage crowd is that SSM is an attempt to “destroy marriage.” This proposal plays directly into their hands. The best tactic for the gay marriage lobby to take right now is to insist that allowing gays to marry wouldn’t have any effect on anyone else’s marriage. We’re not going to get everyone in the country to accept homosexuality full heartedly, but there’s a good chance we can split off the people who don’t like gays, but can appreciate a “you leave me alone, I’ll leave you alone” argument. As soon as you start proposing radical revisions to these people’s established relationships, you’re going to lose them for good.