Proponents of gay marriage see it as a civil-rights issue. One is to take the state completely out of it. If couples want to cement their relationship with a ceremony or ritual, they are free to do so.
Religious institutions are free to sanction such relationships under any rules they choose.
When children or large sums of money are involved, an enforceable contract spelling out the parties' respective rights and obligations is probably advisable. But the existence and details of such an agreement should be up to the parties. And privatizing marriage would, incidentally, solve the gay-marriage problem. It would put gay relationships on the same footing as straight ones, without implying official government sanction. No one's private life would have official government sanction which is how it should be.