The point is that Obama is supposed to represent, as you say, some 300 million citizens, and not just the ones he agrees with. Personally, I think Obama courted the American GLBT vote to get elected and has no intention of doing anything to change the current situation. The appointment of Rick Warren just confirms it.

I will say, though, that if Obama had chosen a minister who said that women or blacks or Jews were inferior and unnatural and not to be tolerated and didn't deserve the same treatment as everyone else, a whole lot more people might understand the anger that GLBT Americans feel over this.