And explicit scenes with Dumbledore already have appeared in fan fiction.
- Harry Potter author JK Rowling announced the character of Dumbledore was gay. In response, my roommate said, "No shit. The whole ing thing is gay."
I also highly doubt that it was a spur-of-the-moment thing. It's obvious that she's invested a ton of time in developing back stories for all the characters of the HP universe, so why would she just make that up on the spot?
The issue never came up in the books, so it was never mentioned.