The broader point (of politics, anyway) is, as you imply, this: Any number of the people that someone like Prager speaks for (though perhaps not Prager himself) wish to make gay marriage an issue precisely because they think it's a winning issue. (I have no opinion on this; I've seen polls that go both ways, to coin a phrase.) In that sense, I too hope that Sullivan stays part of the debate. For all his flaws, and for all his personal interest in the issue, I don't suspect he's playing politics with the issue. The social right, I think, is.
One of the things I respect about Bush is that he's obviously not spoiling for this fight. He has tried to keep his rhetoric well below the level of, say, a Rick Santorum. But, winning issue or not, he doesn't want the culture wars reignited right now. The hard line out of the SJC and the hardline we can expect from the cultural right do nothing for the president, and nothing for John Kerry for that matter. I wouldn't be surprised, in a Bush-Kerry race, by a tacit agreement between the candidates to keep the issue off the table.