First of all. Sirius wasn't gay as mastermoo pointed out. The only reason he didn't have a girlfriend is because the girl he set his eyes on (he won't admit it) was Lily Potter, before she married Jame's Potter. Later on he had to stay in hiding all the time. It's not like a dog can take a woman out to dinner. :/
As for Dumbledore being gay. ... I'm not denying this because of my Christian views. I'll go ahead and say that.
I believe that Dumbledore wasn't gay, and for a couple of reasons. The "proof" that Dumbledore was gay with his buddy (can't remember the guy's name x_x ) isn't proof at all. His "great loss" was the fact that he lost a great friend, and just about the only person he felt was as intellectual as him. He did not adore him "gaily"(sp?), the "adoration" he had for him was merely the admiration of one colleague for another. I mean come on, he spent his whole life with his liberal family who didn't think about things as deeply as he did. HE HAD NO ONE TO TALK TO. Until his friend of course. If you spent years trapped in a box with mice, wouldn't you jump at any chance you had to talk with a human being? When I say liberal, I use it concerning the background his family grew up in.
Another reason Dumbledore wasn't gay, has to do with something Rowling herself said.
"Not everyone likes her work, Rowling said, likely referring to Christian groups that have alleged the books promote witchcraft. Her news about Dumbledore, she said, will give them one more reason."
Why would she say this? If she was fine with Dumbledore being gay, she would have no reason to say it. It wouldn't affect her at all. In fact, I think her publishers coerced her into saying this for the sole reason of more publicity. The statement she made was most likely a side note she made which showed that she cared what the Christian's thought of her book.
You may think me an idiot, but, having seen many "different" things in my lifetime, I have a pretty good insight into the human psyche.