I haven't read any of the Harry Potter books all the way through, so I have no particular investment in finding out that the author thinks one of the main characters in the books is gay. But someone's romantic attractions would seem to be an important part of their character, and if the author didn't see fit to mention this fact explicity in any of the SEVEN books in which this character appeared, why would she do so now? This seems like authorial trespass (note 7) to me.
At this point at which Rowling released the books, she no longer controlled them. And I suspect that Rowling knows this but she's either trying to sell more books, or maybe she just misses that level of control. Regardless, savvy readers should ignore her and make up their own minds about the characters, based on whatever is present in the text.
I do have a lot of respect for Rowling, and the fact that she wrote a series of bestselling books that sold an astonishing number of copies, and appealed to adults and kids. Just prior to the release of the last book I was at the Grove and saw crowds of people dressed as characters, waiting for the book to be released at midnight. That is kind of inspiring. I would be just as interested to find out about Dumbledore's sexual orientation from any of those people who were waiting for the book's release, than I would from Rowling.