I've been thinking a bit about just what it is we're doing here and trying to make sense of it, and I mean that in the most serious way possible. We're talking about a property of a fictional character. Dumbledore is a figure in a story. What does it mean to say that Dumbledore, who does not exist save in our imaginations, 'is' something we don't imagine him to be, something independent of how we think of him? Or any of the other characters? With real human beings that makes perfect sense, but what does it mean of a completely fictional entity?
It seems like a kind of bizarre, even surreal thing to be doing, but here we are, a group of reasonably reasonable people, doing it... so it must make some kind of sense. What could that be? I've been trying to work out a model for what it is we're doing here, and I have something that seems to me to make the activity a reasonable thing to do—something also seemingly bizarre, but which turns out to make a very weird kind of sense, even if no one understands how. So let me try it out on you folks...
We have a work of fiction, and can think of it in a very basic way: a set of entities and a set of situations that those entities enter into. And every time an entity enters into a situation, something emerges about that character. So if Colin McCreevy is part of a crowd going to a quidditch match, the result of Colin's being in that crowd, so far as we're concerned, is at least this: that we can assume he knows what quidditch is. And in general, every situation that a character enters into, every interaction, is in a sense like an experiment which yields certain results about the characters, like measurements in a physics experiment tell us whatever the experiment is designed to tell us: how fast something is moving, how much it weights, what it's coefficient of friction is and so on. So that's the first part of the model: a situation which a character is part of is like an experimental measurement of one or more properties of that character.
But we also know that at the most basic level of physical reality, matter cannot be said to actually have any values for any physical property until these have been measured. The absolutely crucial revelation of quantum theory was that at the most fundamental scale, a particle does not have a value for momentum, energy, location, spin or any of the other basic `quantum numbers' until it undergoes an experimental measurement for some particular property. It's not that it has a value but we can't get at that value. It's not even that it has that value but the measurement in itself changes the value it originally had, so we'll never know what that was. These ideas were the ones that the pioneer generation of modern physics tried to apply to make sense of this incomprehensible, but seemingly infallible theory it had discovered. What emerged from the mathematical foundations of the theory was that the actual state of affairs was much weirder than any of these: the particle does not possess a value for any physical property; rather, it possesses a spectrum of possible values for that property, none of which describe it until a measurement is carried out that yields a specific number. At that point, the probability of one of those values (even one of the lowest probability possibilities) becomes 1 and all others become 0.
My idea is that properties associated with fictional characters are similar. In the absence of an actual measurement—a situation which yields a`fixing' of the value for that property—the character's height, ethnic identity, taste in beer, sexuality, or medical history—anything at all about them, in fact—is exactly the same as that of a quantum-level particle which has not passed through an experimental apparatus designed to measure a certain dynamical variable such as energy, position or spin. The character simply does not have a value for that property; rather s/he (and this exends to inanimate objects as well) has a spectrum of possibilities associated with any of those kinds of personal properties. Unless the character enters into a situation which yields a measurement, so to speak, for that property, we have nothing but a spectrum of probabilities, a kind of smear of ghostly values-in-waiting none of which can be said to hold of the character at that point. This obviously is not true of real human beings: someone can be a reformed compulsive gambler even if not one person in the world is aware of it and possesses no information about the person in question. But fictional characters exist only in the same way that particles at the quantum level exist: till we have interactions which measure something about them, they aren't there, and apart from the measurements we take, they do not possess values for personal properties. The author constructs situations which the characters are subject to just as the physicist constructs experimental apparata that matter at various scales is subject to. The results are similar, and so are the cases of properties that are not measured.
Now in a sense, I think the point that Rick and I are agreed on here is that none of the situations in the saga yields a value for Dumbledore's sexuality; I'm also reasonably sure that none yields a value for whether or not he's a carnivore, or an opera fan. If so, then he isn't gay, and he isn't straight, just as he's neither a carnivor nor a vegetarian, and neither an opera-lover, an opera-hater, or a complete neutral on the subject: his status with respect to all of these parameters belong to his probability spectrum... but none are true of him, none constitutes a valid description of him. Like Schroedinger's cat, we won't know until the box is opened, which collapses the wave packet, as they used to say. Only, since the saga has come to an end with the box still closed, there's never going to be an actual value determined.
And whatever shaping power the idea that AD is gay may have had with JKR in developing the character—and I agree with tellner, that could very well have led her to develop his demeanor and way of relating to people in a particular way that is reflected throughout the novels—it doesn't look as though any of the situations he's in in any of the books actually forces a measurement. Maybe someone will turn one up, and then it will a different story. But for the present, I think it makes way more sense to say that that AD doesn't have a sexuality.
She reminded me a lot of the professor who was our mentor in teaching women's self defense.
I am not. But you can think it or inply and it is ok with me. 
