In a March 9, 2008 interview JK Rowling admitted that Dumbledore is gay and was in love with a wizard named Grindelwald whom he later defeated in a duel.
She said and I quote, "I had always seen Dumbledore as gay, but in a sense that’s not a big deal. The book wasn’t about Dumbledore being gay. It was just that from the outset obviously I knew that he had this big, hidden secret and that he flirted with the idea of exactly what Voldemort goes on to do, he flirted with the idea of racial domination, that he was going to subjugate Muggles. So that was Dumbledore’s big secret.
He’s an innately good man, what would make him do that? I didn’t even think it through that way, it just seemed to come to me, I thought, ‘I know why he did it. He fell in love.’
And whether they physically consummated this infatuation or not is not the issue. The issue is love. It’s not about sex. So that’s what I knew about Dumbledore. And it’s relevant only in so much as he fell in love and was made an utter fool of by love. He lost his moral compass completely when he fell in love and I think subsequently became very mistrusting of his own judgement in those matters so became quite asexual. He led a celibate and a bookish life."