Making it clear that I don't agree with Andrew Sullivan on the particulars of this issue, a couple of useful links from his blog:

First, a demonstration of how anti-gay fear-mongering is losing its value to the GOP. (the heart of that is essentially Obama's position, not Romney's)

And between these two discussions equating Obama's stand here to LBJ's stand, I agree almost completely with Jonathan Chait's description:
The Johnson comparison is pretty nuts. Johnson’s primary contribution to the civil-rights cause was to pass a federal law. The need to pass a federal law was the central problem of the civil-rights crusade, and the barriers to passing such a law in Congress had been the main impediment to the cause for two decades.
Obama has not passed or even proposed a law. The gay rights question remains a state matter. That’s exactly why Obama was able to maintain his obviously insincere formal opposition to gay marriage for so long — because his formal endorsement had no practical consequences.
Now, symbolically, it matters. Obama’s imprimatur will probably hasten Democratic loyalists who don’t accept gay marriage — and there are a lot of them — into more readily accepting it.
Sullivan responds, "But he has withdrawn the DOJ's defense of DOMA." But that does nothing except emphasize the idea that it's an issue that should be handled at the state level. Which is exactly the defense that's been used to rationalize every single state referendum to write discrimination into states' constitutions.

Of course it's nice when a person says, "I believe gay people have every right to be married as heterosexual couples." Unless you do something to back it up, though, there's no appreciable difference between saying it on ABC News and saying it in a Facebook status update.

For years I believed that debating the topic with strangers on the internet was constructive, because at the very least I was helping fight misconceptions. And over the years, the "it's wrong and unnatural!!" comments slowly started to dwindle; for some reason, people who have no problem spouting homophobic sentiment absolutely hate being called homophobic. But it didn't go away; instead it's turned into one of two things: 1) "Look, I'm totally in support of gay rights. I just believe that... some variation on the argument that's totally not in support of gay rights." or 2) "This is what I believe, and I'm entitled to my opinion." That's what passes for civil debate these days. Obama's statement is a classic example of (1).

And putting so much emphasis on its symbolic value as a statement from a sitting president is just a facilitation of (2). Changing "hearts and minds" simply doesn't work. If we accept that the opinion of the person saying "I'm in favor of gay rights!" makes a difference, then we have to accept that the opinion of the person saying "I can't condone it" is equally valid. It's not. Somebody has to step in and say no, your opinion doesn't trump my opinion when it comes to my equality. Because it's not about opinion.