It's high time this idea hit the liberal blogosphere. The gay community has been talking about getting the gov't out of marriage for a long time, yet for some reason this is the first time I've encountered it on a blog. (Perhaps last week's West Wing helped get it out there.)
In addition to the sly political strategy laid out at the link above, it's also worth noting that getting the gov't out of the marriage business helps avoid that pesky Separate but Equal clause, which someone is bound to pick up on once they realize it's marriage for heterosexuals but civil unions for gays.
And aside from all of that, let's also remember that, plain and simple, this is the right thing to do.