He's gay. Big whoop. I actually figured that before the 'unveiling'. I won't say that it's obvious, but it's there if you look. And you're right, it is a kids book, so it doesn't really focus on the relationship side of things other than making out, even with the hetro relationships.
It's not like she deliberately led us to believe that he was straight. Come on - he's one of a few of the elder wizards/witches that don't have a spouse. There was always going to be a chance that someone was gay (I'd pegged Charlie Weasley and Dumbledore myself. 1 out of 2 ain't bad.)
Besides, he isn't going to be mooning over every male in the immediate area, because being gay is about loving someone of the same gender. Love strikes where it will, people. And the one he loved is dead. That's not to say that he doesn't feel love for others, but no others are mentioned.
I always figured that Dumbledore and Harry's relationship was that of Mentor/Student + Grandfather/Grandson. Innocent.
Yeah throughout the text there were subtle hints where Rowling would just say dumbledore "prepared his robe and traversed to the council" and similiar things. There are also several subtexts, Dumbledore often mentions the number 5 in his musings, which can be as simple as the random noting of the time etc.
There's also strong hints towards his thelemic philosophies, and undoubtedly as the headmaster of the most prestigious school of magick entertained tantric rites, and combining these two predicts a necessarily homosexual, or more specifically bisexual sexuality.
Even in the earlier books his devience can be seen. It once talks of him drinking a "strange potion," and the season was set elsewhere as spring, so this was undoubtedly a challice prepared specifically for the spring equinox, which is usually a mix of blood and semen.
There are other really obvious things, and it's clear even from the movie. I knew it instantly just from his name:
Albus Dumbledore. Albus is obviously derived from Aldus, in reference to the author "Aldus Huxley." Swapping the d for a b, it's symetrical opposite in the middle of the name signifies a reversal of Huxlian attitudes (Huxley is most famous for his book "brave new world," so this opposition in itself is representative of his adherence to the old ways (homosexuality has existed since evil men could rub their dicks together)) and also a reversal of the staunch heterosexual that Aldus was (though this is questionable in doors of perception). Dumbledore is extremely significant. It is the phonetic twin of the kabbalic "Gjomblei-gjore" but made with exactly 10 letters. This is fairly obvious in itself so I won't go too far into these details.
There's numerous other references which make it very clear the family of magicks that Dumbledore subscribes to, mostly crowlean, with numerous references to thelema (as mentioned earlier, the golden dawn, and Rowling is even so blatent as to have a spell "Avada Kedavra."
It's pretty easy to pick this stuff up if you've escaped what me and my friend call "the loop of mediocrity" and ascend above the level of such mindless childrens books readers to find the ultimate truth of our advanced philosophy of situational relativism.