I'm afraid that too many people in the anti-gay-rights movement camp will turn it into, "Wait, now you're telling me that I can't get married?" (or, worse, "Now you're telling me that I'm not married anymore?"

It's true-- one of the major arguments raised against people in favor of same-sex marriage is that we're trying to "undermine the fundamental nature of marriage." Which I don't think has been done by gay marriage, per se-- that was done by women being able to own property and read and use birth control. But this, as a move, would be feeding right into that line of argument, and that is a serious flaw in it.