The "truth" about the character is whatever the AUDIENCE reads it as. There is nothing in the text besides the fact that Dumbledore was never romantically involved with anyone to substantiate a reading that he is gay. That fact, of course, doesn't substantiate that he was straight, either, just that he never had a romantic relationship with anyone that we know about.
If you need to tell your audience a basic fact about a main character, you've done a bad job as an author. Think the Dumbledore/Snape genre is going to get WAY out of control with this revelation though. Hmmm, maybe it was Snape/Dumbledore with Dumbledore acting as a proxy for Lily. Who knows?
I guess all this question/answer nonsense bothers me from a literary perspective. The story ought to tell itself and if it doesn't, that means that the story is either poorly written or the fact is generally not relevant to the course of the story and the audience may decide on its own what they think. Though I don't buy all of the tenants of "New Criticism," I think their stance on intentional fallacy is right on the money. (
Intentional Fallacy) Given ambiguities in a work of art, the artist is just as welcome to anyone else about what the art says or means, but his interpretation should not be privileged without textual backing.