It makes a certain amount of sense (and if you haven’t read Harry Potter and the Deathly Hallows and don’t want to be spoiled, don’t click below..)
…because, supposedly, he was in love with the evil wizard Grindelwald, and it’s established in that book that they were really close, but Dumbledore being Dumbledore, he let his emotions take over and trusted him a bit too much. Dumbledore’s greatest strength and his greatest failing is love, so it’s not too much of a surprise that he was in love with Grindlewald. The books never dealt with Dumbledore as a sexual being at all – gay or straight – so the fact that this was at the heart of his character all along without being an issue is a pretty profound statement of tolerance, methinks. And, as tolerance has always been one of the lessons of the series, it even makes sense thematically.
No wonder the Dursleys were so freaked out by him at the beginning of Half-Blood Prince. Anyway, Fandomwank, as always, is on top of the (inevitable) controversy this has caused in fandom. (From the “Don’t You Wish You’d Thought Of That Dept.”: they’ve gone and titled the post “I Love My Dead Gay Headmaster.”)