I must be the only one who thinks this makes sense. Obviously his sexual preference was never mentioned during the books explicitly -he's seen mostly through Harry's eyes as a mentor, plus he's an old man, his sexuality isn't an issue by then -but the young relationship with Grindlewald seemed to me like a teenage infatuation. I've always thought of Dumbledore as someone who loved and lost and, sexuality-wise, I think he's never showed any signs of heterosexuality. Point is -I don't understand how dumbledore being gay will hugely change anyone's reaction to the books -he's written quite consistently as a character who could possibly be gay. Having it confirmed makes a difference, but doesn't undermine what came before it.
As to the people who are appalled that a gay man is portrayed as a mentor to lots of school children (not necessarily directed at someone here, but it's something which I've been hearing a lot of) -can you hear yourselves? Are you seriously suggesting that there is something wrong with the idea of a gay man being a good teacher/mentor? Why on earth should that make a difference?