Literary Agent Nathan Bransford had a very interesting You Tell Me today. In today's post, he brought up the news about how J.K. Rowling revealed in an interview that Dumbledore is gay. He also pointed out a follow-up article in the NY Times that essentially said that her revelation is irrelevant, because it's not very supported by her text (among other comments -- that's an interesting article that you should read before finishing this post).
Nathan then posited the question to his readers: who owns fictional characters? The author, the readers, or some combination? I think that Orson Scott Card said it best in his introduction to his 1991 edition of ENDER'S GAME:
"The story of ENDER'S GAME is not this book, though it has that title emblazoned on it. The story is one that you and I will construct together in your memory. If the story means anything to you at all, then when you remember it afterward, think of it, not as something I created, but rather as something that we made together."
I read that many years ago, and it has stuck with me ever since. It bothers me a lot that Rowling announced that Dumbledore was gay like she did, and I think readers are certainly free to interpret his character as asexual or however else they want to, as long as it is consistent with the (mostly scant) textual evidence regarding his sexuality.
Now, if Rowling had made this revelation in due course as part of one of her novels, that would have been a different matter. She's perfectly free to do whatever she likes with her characters in the context of her writing, and that's a license held exclusively by her as far as I'm concerned. If she had revealed him to be gay in the context of the story, I would have been fine with it, although admittedly I prefer the vision of him as a great wizard above sexual concerns, as the times article notes. But it would have been her choice, since it was her work.
I think it's rather cheap in general for authors to explain backstory outside of their actual novels. Certainly it's not a problem with minor details such as specific age, birthplace, etc, that just wouldn't come up in the story -- but anything that seemingly flies in the face of textual evidence, or comes as a shock, or even seems a stretch based on textual evidence (as this does) shouldn't be revealed outside the narrative, in my opinion.
This would be like if George Lucas had revealed in an interview that Darth Vader is Luke Skywalker's father, after only making the original movie. How horrible would that have been? Certainly there was foreshadowing for that revelation, but if it had been revealed outside the narrative of the movies it would have seemingly lost all truth and meaning.
I believe that the author has the exclusive right to do anything they want with their worlds and characters, but only in the context of their narratives themselves. Once those pages are in the hands of the readers, the readers are going to make their own inferences and come to their own personal understanding, right or wrong. A later work can certainly cause readers to revise that personal understanding -- all the best sequels do this to some degree -- but this is expanding the story, not clarifying it in interviews or endnotes. At least, that's my take.