I'm pretty happy with Dumbledore, myself. I mean, the most powerful wizard in centuries? The hero's mentor and most important father figure? (Yeah, I know, I still want Sirius to have been Harry's most important father figure, but that's me and my little world.) Deathly Hallows demonstrates that Rowling saw the books as being nearly as much about Dumbledore as about Harry (again I would have liked them to be more about Snape than Dumbledore but again that's another visit to Amy World), and for her to have crafted that particular character knowing that his sexuality leaned that way--well, yeah, I am pretty happy about that choice.
In fairness, sure, I'd pick Harry first. God, just imagine. Harry ends up kissing Draco Malfoy instead of Ginny Weasley. I'd have died of an ecstatic brain hemorrhage. Of course, that would have required a rewrite of the last two books--all to the good, because the Harry/Ginny romance remains gag-worthy (and not, repeat, NOT because it's het).
But I wouldn't pick Snape because I wouldn't want Snape to be "the gay one" all alone. It would suck for me, thinking, "Oh, great--Snape, the one who's sullen, dark, sneaky, grudge-holding, unfair, hated, picked on--let's get a few more negative stereotypes in here, shall we?" Shylockian, I would have called it. Whereas outing Dumbledore--Dumbledore, despite his not-so-pristine past and manipulative ways, comes off as fabulous. Stereotype, yes, but at least a positive one. Still stands on its own outside of that closet and happily kicks your ass if you protest. I mean, yes, I love Snape and imagine him as gayer than a treeful of monkeys but I want him to be outed with somebody, if that makes sense. In fact, by outing Dumbledore, it does call his intimacies with Snape into question. But by implying "Snape with Dumbledore," homosexuality would not be read as just one more dark aspect of Snape's character, done in this way.
Lupin, on the other hand, would have been fascinating. Imagine Rowling saying, "Lupin was not merely conflicted about his romance with Tonks because he thought he was too old and too poor." Imagine what that would have invited. Lupin got married despite having other leanings, became resigned to it after his son was born because it had brought at least one wonderful thing into his world. The idea rankles because it wears a little sheen of "reproduction good, so maybe gayness not so good" upon it--but it invites one to see bisexuality and homosexuality within the text even where there appears to be happy heteronormativity between couples. Loaded. Hugely loaded. But fascinating.
I feel about Sirius the same way I feel about Snape--don't let him be gay alone; put someone with him and I'd be happy. (Like Snape.) Yet it's not because of Sirius being thought of as a dark character, though--I wouldn't have liked the idea that he was conveniently killed off so that no one would have to resolve the idea of him being both gay and a parent/older brother/best buds figure to Harry. As if it were punishment. If he'd been paired with someone--Snape, Remus--then devoting that degree of depth in the story to two gay characters might have erased those "punishment" feelings. So it isn't just a good guys/bad guys thing here, in terms of whom to de-closet and what would avoid the negative stereotypes. Draco could have been gay and I wouldn't have winced. Lucius? Heck, Lucius would rival Dumbledore in fabulosity.
But I do like that it's a good guy, and a hugely significant character, and the most powerful wizard in ages. Yeah.