here's the thing. Rick Warren isn't going anywhere. Obama can make up for this by doing a few things.
1: repeal the Defense of Marriage act so the federal government will recognize same sex marriages in states that allow them.
2. repeal "don't ask, don't tell"
3. Pass the Employment Non discrimination act so you can't be fired for being gay (we already have a law like this in New York, but other states don't)
these steps will matter more than who speaks on the 20th. It doesn't excuse it but actions speak louder than words from a radical cleric.