Did Dumbledore, who believed in the prevailing power of love, ever fall in love himself?
JKR: My truthful answer to you... I always thought of Dumbledore as gay. [ovation.] ... Dumbledore fell in love with Grindelwald, and that that added to his horror when Grindelwald showed himself to be what he was.
http://gothamist.com/2007/10/20/jk_rowling_to_n.php
That website has a link to a transcript of the evening...That wasn't the entire answer to that question, but I didn't really want to put the whole thing in. My thought is just that she mentioned him being gay first to lead into the Grindelwald thing, but only the "he's gay" part was reported.