http://www.the-leaky-cauldron.org/2007/ ... cores-more
First, the biggest revelation of the night came when Jo revealed to her audience the fact that Albus Dumbledore is gay and had fallen in love with fellow wizard and friend, Gellert Grindelwald. This elicited a huge reaction and prolonged ovation. So much so, it promoted Jo to say:
"If I had known this would have made you this happy, I would have told you years ago."
The question was: 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. To an extent, do we say it excused Dumbledore a little more because falling in love can blind us to an extend, but he met someone as brilliant as he was, and rather like Bellatrix he was very drawn to this brilliant person, and horribly, terribly let down by him. Yeah, that's how i always saw Dumbledore. In fact, recently I was in a script read through for the sixth film, and they had Dumbledore saying a line to Harry early in the script saying I knew a girl once, whose hair... [laughter]. I had to write a little note in the margin and slide it along to the scriptwriter, "Dumbledore's gay!" [laughter] "If I'd known it would make you so happy, I would have announced it years ago!"
Jo also said after revelation: "I had to give you something to talk about for the next 10 years...Just imagine the fan fiction now."