The numbers have changed since 2004 -- when anti-gay Republican ballot initiatives drove GOP get-out-the-vote and substantially helped Bush win his first election (a partisan Supreme Court selected him in a putsch for his first term, and I won't ever forget that) -- but the numbers may not have changed enough that this issue cannot still function as a wedge around the suburban and elderly edges in ways that might impact tight Congressional races. I do still happen to think it matters more to regain the House and retain the Senate so that Obama can actually do anything remotely like what needs to be done than it is to have the President publicly admit what everybody with sense knows he actually already believes. But I have to assume the campaign gamed this out and up side outweighed downside all in all -- quite apart from, you know, the principle of the thing.
I do hope that certain lgbt activists will put away their "Obama = Fraud" posters (speaking of the administration under which progress for queer folks has leaped forward unprecedentedly as a matter of fact) and will put real energy into Obama's re-election because he has taken this principled stand under pressure of events. I personally think this pressure was misplaced and a distraction from the more urgent jobs crisis and regressive taxes foregrounded by Wisconsin and Occupy. And I think this not only because I also happen to think the institution of marriage is a vestige of human trafficking and a mirage of Hallmark card ideology (a whole bundle of important substantial legal affordances connected to the institution complicating this stance notwithstanding), and my partner of ten years agrees with me. Still, soon I hope to be able to critique marriage as a queer feminist from the position of someone willfully refusing the status rather than as someone always already excluded from it, much the stronger position.
What is done is done. Obama has done the right thing rather than the calculated thing, he has opted for the principled stance over triangulating weaseling, and the President's affirmation provides a righteous countervailing force to the ugly North Carolinian vote to make already illegal gay relations even more extra special totally illegal because they're assholes thing. I wouldn't much care if this hadn't come up, but I'm happy our President affirms in yet another way the principle of equity-in-diversity on which the democratic project thrives.
By the way, Obama's statement is only metaphorically an instance of "evolution," of course -– and one more way in which Democrats differ from Republicans is that more Democrats know the difference and affirm the reality of the literal kind, too.