I was off the mark, I know. I blazed through the first four books five years ago, in the course of about two weeks, at the request of my younger brother, but hadn't followed the story before, nor since, then. And Dumbledore is immaculate in those earlier books.
Turns out, though, Rowling mixed it up after that point. To where I'd read, each of the characters was clearly either a good guy or a bad guy (with the exception of Snape, a personal favorite alongside Percy). That's a prudent strategy in the beginning for an unknown author--don't make the thing burdensomely confusing. There are an endless number of aspiring writers who create novels with universes that are entirely too complex to take in all at once and so remain forever in obscurity. Once preeminence is establish the complex, entangling alliances and animosities can then be flushed out (the Warcraft series has followed this universal progression with great success as well).
By the time it was made known that Dumbledore--rather than being the asexual professor Rowling portrayed him to be--was gay, illumination on his past revealed a few skeletons tucked away. Ultimately, his penitent desire to apologize to his parents revealed that even in later life he remained fallible. Further obliterating my initial suspicion, Rowling showed that his unrequited love for another man was his Achilles' heel:
Rowling said Dumbledore fell in love with the charming wizard Gellert Grindelwald but when Grindelwald turned out to be more interested in the dark arts than good, Dumbledore was "terribly let down" and went on to destroy his rival.Had he been been stoic in the face of this desire as he'd been in the face of so many others, he would've knocked off Grindlewald several years earlier.
That love, she said, was Dumbledore's "great tragedy."
"Falling in love can blind us to an extent," she said.
Realizing it is the fantastical Harry Potter series, Dumbledore is not the most credulous choice for a homosexual character. He wasn't flamboyant, conspicuously flirtatious, (with both men and women), gesticulative, or particularly focused on dress as 'ladylike' gay men (those whose sexual preference is evident from a mile away) tend to be. Nor was he sardonic, cynical, haughty, and equipped with a slightly misanthropic streak like 'the guy' gay guys often are (ie, Waylon Smithers when not dealing with Burns, or Squidward from the Spongebob series).
The most reasonable candidate would've been Snape, who fits 'the guy' definition to a tee. Well, that is, until it is apparently later revealed that he is a Sydney Carton-like character whose love for Harry's mother leads him to fidelius devotion to Harry, even though he despised Harry's father and despises the son to his dying day.
Yikes! That means my initial fear might still come to fruition. Snape, perhaps the series' greatest character, is nobly driven by his love for a woman. Dumbledore, who may share 'the greatest' award, is flawed only in his love for another man.
If it teaches young readers that an innocuous revelation about a person's character should not retroactively change their opinions of what they appreciated about the person before, splendid. Why not add that his grandfather was from Zimbabwe and his grandmother from Calcutta while at it (or maybe Tehran, so a charge against Rowling of pining for the old empire doesn't come up)?
That potentially positive message aside, the revelation is going to further take away from Dumbledore's larger-than-life character, for better or worse. Positively, it might be looked at as humanizing him.
It will do so not because he's gay per se, but because he has an explicit sexual orientation and a drive that he acts upon. Sexual desire is inherently selfish, and consequently it generally has a moderating effect on both the good guys and the bad guys, pulling them both closer to the neutral center by diverting attention from their more purely good and purely bad objectives (as what many of those in the 'Jesus had sex with Mary Magdalene' camp are trying to do, by diminishing the man from Galilee's putative holiness and bringing him down to a more human level).