Yes, it is true. According to the story I read, she had told the writer of the screenplay for HBP that Dumbledore was gay because there was mention of a female love interest in the screenplay and she had to set them right. Having done that, she might as well have made it public, since it would have eventually come out anyway--she could control when and how it was done.
Throughout the series, we know Dumbledore as a tireless and seemingly single-minded fighter against Dark Wizards (at least from Harry's perspective; we do have to remember that Albus is also a great teacher, an alchemist, and important theoretical Wizard--remember the tools and instruments he made himself). But how did Albus become that way?
We learn much about this in DH, with Arianna's story, Aberforth's story and the story of the friendship between Dumbledore and Gellert Grindelwald. In reading DH, it was clear that Grndelwald seduced Dumbledore on a number of levels; intellectually, ideologically, and, by JKR's new revelation, emotionally. We don't know if that seduction was sexual as well, and I'm not sure it is important for us to know.
The revelation completes the picture for us of a man who felt betrayed, injured by one he loved, ashamed at how far he went from rational thought and behavior, how far he really was from what he thought he was.
The up shot is that we now know more completely Dumbledore's motivations for fighting Dark Wizards--his shame at his own tangential contribution to Grindelwald's career, his reluctance to fight him, and, when Voldemort rose, his determination to not to repeat his prior mistake of inaction and to fight Voldemort from the start.
So in summery, It was part of the way JKR viewed Dumbledore and understood him. I don't think it was something that would have come out voluntarily--more likely something that would have made a scholarly article 50 or 60 years from now after she was gone and literary scholar had access to her full notes (assuming she left them to be investigated).
BTW, there is some inkling of this in DH, where Rita Skeeter talks about devoting a chapter in her book about Dumbledore to the Dumbledore/Harry relationship. This is not to say that there was anything improper--but that JKR did put out a hint.
wl