So Rowling outed Dumbledore. This post isn't really about that though.
This post asks the question; does the author's opinion matter any more than anyone else's once the story is out?
Since the last book has been written, there is only the written evidence to go by. If Rowling thought of Dumbledore as gay while she wrote the stories, that's fine, but hardly relevant. I think the real issue is, if the reader thinks of him a gay, then he is. If not, then he's not. The authors opinion matters not a bit once the story is done.
The reader brings a great deal to the story and it behooves writers to remember this. I've read many stories -- especially ones that are still in the critique stage -- where I believed the main character was one gender only to find out part way through that I was wrong. Since the story was not complete, I had to go with the author. But had I read a published story where I believed the a character was a Dalmatian then the author later told me it was a Siamese cat, why should I care about that. For me the character was a Dalmatian. I think in a case like this, my opinion matters more. I am the reader. There is nothing there for me to go by other than what is written down. The authors objective may be achieved or completely misconstrued.
Half of a book's intent is written and half of it is read. The author should not go around trying to change that after the fact.
Saying Dumbledore was gay is sort of like saying a character is blonde when no mention of his hair is ever given in the text. If the reader imagined him as bald, then he was bald. The author's opinion is just that, once the story is published, opinion.
I admit I never really thought about Dumbledore's sexual preferences, so it matters little to me, but it seems wrong to try to change the story after it's already on paper and out the door.