I'm coming back to apologize, because I see from a transcript quote that JKR didn't use the word "unrequited," but says simply that Dumbledore fell in love with Grindelwald and then felt "terribly let down" by him.

So, I'm imposing my own prejudices onto JKR here, which is very unfair of me. But I still think that she didn't need to make Dumbledore's sexuality subtext.