This is just stupid. The Harry Potter series is DONE. The plot never turned on this obscure piece of trivia, and to "reveal" that Dumbledore was "gay" is just idiotic.
I've seen this many times, when an author brings in a gay character just because they can. It's like they're saying, "Ooo, look, I'm cool, I'm politically correct, I'm not a homophobe!" when there is no relevant connection to the plot.
It's irritating, because it usually throws me out of a rollicking good story - a cardinal sin for an author. Authors, please don't do that to your readers! If it doesn't advance the plot, if it isn't truly important for character development, then it doesn't belong in the story.
I've always thought that, if the author can't restrain him/herself from this sort of stupidity, then the editor who allows it to slip by ought to be shot. (Yes, that was hyperbole. But not by much.)
Of course, Ms Rowlings, IMO, compounds the error by mentioning it after the completion of the series when it is an incredibly irrelevant point. Honey, you're already a successful and influential writer - you don't have to make yourself more "hip."