If you haven't heard the story behind it yet. read here:
My two cents:
Making a major character in your novels gay as a "prolonged argument for tolerance" doesn't really work if you have to explain to readers months after the last book in the series ends that he actually was gay.
I mean, I guess it didn't really come up in the series, but as far as I got in the books there was no indication as to Dumbledore's sexuality at all. Not that the opportunity presented itself, but still.
I support broadening people's minds on this subject, but this wasn't as big a weapon for gay rights that J.K. Rowling might think it is.