I agree with you on this. There are a lot of relationships that totally slip under the wire, like Dean/Ginny, or Cho/Cedric.
We all see characters in a certian way in our heads, and I'm cool with her having Dumbledore be gay for her, and have this tragic relationship with Grindlewald that could have brought him down. While I would have liked to have this fact in the books somewhere, it really is a non-issue for the story. The fact that he was gay would have nothing to do with the draw of power. Or even the lose of a good friend. I don't need to be a lesiban to be impacted by a relationship between my best friend and I.