Of my remark, in response to J.K. Rowling's "by the way, Dumbledore's gay" announcement, that "a writer confident in her powers wouldn't feel the need to announce details like this," Neil Gaiman - yes, that Neil Gaiman - writes:
All that tells us is that Ross Douthat doesn't write fiction.
You always wind up knowing more about your characters than you can get onto the page. Pages are finite, and the story isn't about giving you all the information about everyone in it any more than life is. Things the author knows about characters (or at least, strongly suspects -- it's never really real until it hits the page, because the process of writing is also a process of discovery) that don't make it onto the page could include the characters' backstory, what they like to eat, the toothpaste they use, what happens to them after the story is over or before it began, and what they do in bed. That something didn't turn up in the books just means it didn't make it onto the page or wasn't relevant to the story. (Or even, it made it in and the author cut that scene out because it didn't work. One of my favourite scenes in Anansi Boys went because it made the chapter work better when it was gone.)
(I remember being astonished when I learned a few years ago, from an obituary, that two teachers I'd had as a child were a same-sex couple. Mostly astonished because at the age where they taught me, I didn't imagine that teachers had romantic lives, or were even entirely human; and learning that they were a pair reconfigured everything I knew about them, which wasn't very much.)
Neverwhere has two gay characters who are Out, as far as the book is concerned, and one major character who is gay but it isn't mentioned, simply because that character was one of many people in that book who don't have any sexual or romantic entanglements during the story. So it's irrelevant.
... And, truth to tell, sexuality tends to be such a minor thing, if you have several hundred characters running around in your head. You know more than you've written. One of the characters in Wall in Stardust, for example, is not what he is pretending to be in a way that has nothing at all to do with sex, although the clues are all there in the book, but if I don't do another story set in Wall you'll never find out who he is, or even why he's interesting.
I think the crucial question here is whether Dumbledore being gay is just a "a minor thing," just one of many quirks that the reader doesn't need to know while reading the books but might be interested to discover after the fact - or, as with Gaiman's two lesbian teachers, whether it's something that "reconfigures" everything we thought we knew about the Hogwarts Headmaster. I would submit that it's closer to the latter than to the former, given the role Dumbledore plays in the saga and the significance of his varied relationships to all the other players in the story; it isn't quite the equivalent of Rowling never giving the reader any clue that Snape was in love with Lily Potter, only to mention it in passing at a public appearance in 2009, but it's closer to that sort of thing than to, say, a piece of interesting trivia about how Minerva McGonagall once dated a guy from Slytherin. I'm not saying that every piece of information about every character needs to be spilled out on the page; I'm not even saying that one should be able to know, from reading the books, that Dumbledore is homosexual. I'm just saying that a writer with confidence in her powers would write that sort of important detail into the story in such a way, whether explicit or implicit, that she didn't feel the need to explain it after the books came out. (I'm trying to picture Melville patiently explaining to the Illustrated London News that yes, Claggart did have a thing for Billy Budd - or Proust telling a Paris audience: "Don't you get it? The narrator's gay! Albertine's a guy!")
But the larger problem is that by saying that Dumbledore's gay, Rowling opens up a whole host of questions that are deadly to her world-building exercise. The wizarding world only holds together so long as the reader accepts that it's a cracked-mirror version of our world, rather than an internally consistent sub-creation. Thus, for instance, it isn't odd that a society so invested in matters supernatural, in which people constantly brush up against various manifestations of the afterlife, lacks any religion at all - not because, pace Christopher Hitchens, Rowling wants to suggest that a society can be moral without God, but because keeping organized religion out of it lets her use the wizarding world as a Narnia-style commentary (though not to nearly the same degree) on theological matters in our own. And similarly, it isn't odd that the wizarding world doesn't seem to contain any patterns of sexual behavior except strict celibacy, high school crushes, and lifelong monogamy, because Rowling has created a world for young readers in which those sort of adult issues are handled obliquely and/or allegorically. (To steal an example from Mark Shea, her treatment of Remus Lupin's werewolf issues screams "AIDS metaphor.") But the instant you say "Dumbledore's gay," you open yourself up to critiques like this one:
I had always given the Potter books a pass on the lack of gay characters because, especially at first, they were intended for little kids. But particularly with the appearance of the long, violent later books, Rowling allowed her witches and wizards to grow up, to get zits and begin romances, to kill and die. It seemed odd that not even a minor student character at Hogwarts was gay, especially since Rowling was so p.c. about making her magical creatures of different races and species, incomes, national origins, and developmental abilities. In a typical passage, the briefly mentioned Blaise Zabini is described as "a tall black boy with high cheekbones and long, slanting eyes." Would it have been so difficult to write in a line in which Zabini takes the exquisitely named Justin Finch-Fletchley to the Yule Ball?
A week ago, I would have called this somewhat silly, but now it seems like a much fairer question - and one that breeds other questions, which the decidedly un-erotic nature of the Potter books had kept safely offstage. What's the wizarding world's take on sexual morality? Has there been a wizarding world sexual revolution? If not, why not? What do wizards use for birth control? Are all gay wizards in the closet? Why does there seem to be no premarital sex at Hogwarts? And so on and so forth. (The same slew of questions, on a different front, would have been provoked if she'd suddenly announced that Snape was Jewish.) The next thing you know, you're pointing out that the wizarding economy doesn't really make much sense, either ... and suddenly, the magic starts to leak away.