Nevermind the practical results; gay marriage isn’t an issue of liberty at all. No one’s stopping them from having sex, cohabitating, vowing to spend their lives together, raising children, etc. What they’re asking for is the state to intervene and grant them special status. Which has nothing to do with liberty.
That was my point, which I don't seem to have made as clearly as I might have. Two men or two women or some other conglomeration can have a wedding now, in some churches, or hold a ceremony of their own choice. What they can't do, in most states, is force other people to recognize them as "married" or to recognize their relationship as the moral and social equivalent of traditional marriage.
It is, as you say, the opposite of a pro-liberty position.