Originally Posted by Martian

I think this is the key to it. A good author will frequently create a full backstory to a character in the process of creating a believable, well-rounded individual. Much of that backstory will never make it into the main narrative because it's not particularly relevant to the story. Dumbledore's orientation is only relevant in a peripheral sense, as it helps to explain why he was so tolerant of Grindelwald's obvious tendency towards (shall we say) the unsavoury. It is important, I think, to also make the distinction of Dumbledore at 17 versus the Dumbledore the readers are familiar with; at that young age he wasn't possessed of the vast experience and wisdom that so informed his character, which lead to him making some poor choices with dire consequences. For all Harry's exclamations that he and Dumbledore were the same age, Harry also makes poor choices informed largely by the same flaw, if one could call it such. Certainly nothing to the same magnitude, but then Harry's early life would prevent that I'd think.

I'm not particularly surprised by this. I also don't care very much. It doesn't really impact the character significantly, or the overall story.