It's not just some marginal figures who are coming around to the idea but now some serious legal scholars. See this article from Time Magazine:
In a paper published March 2 in the San Francisco Chronicle, two law professors from Pepperdine University issued a call to re-examine the role the government plays in marriage. The authors — one of whom voted for and one against Proposition 8, which ended gay marriage in California — say the best way out of the intractable legal wars over gay marriage is to take marriage out of the hands of the government altogether....
Instead, give gay and straight couples alike the same license, a certificate confirming them as a family, and call it a civil union — anything, really, other than marriage. For people who feel the word marriage is important, the next stop after the courthouse could be the church, where they could bless their union with all the religious ceremony they wanted. Religions would lose nothing of their role in sanctioning the kinds of unions that they find in keeping with their tenets. And for nonbelievers and those who find the word marriage less important, the civil-union license issued by the state would be all they needed to unlock the benefits reserved in most states and in federal law for married couples.
"While new terminology for all may at first seem awkward — mostly in greeting-card shops — [it] dovetails with the court's important responsibility to reaffirm the unfettered freedom of all faiths to extend the nomenclature of marriage as their traditions allow," wrote Douglas W. Kmiec and Shelley Ross Saxer. Kmiec voted for Prop 8 because of his belief in the teachings of the Catholic Church and his notion of religious liberty but has since said he thinks the courts should not allow one group of Californians to marry while denying the privilege to others.
As I recently wrote, the proposal does have a "Madisonian" logic to it.