My understanding is that he was initially gay and that she decided to not state it explicitly while writing, maybe because it was still technically a children's series, or her editor told her, or whatever.
I hadn't looked at it from that angle beforehand, but when the drama came out and the idea was put in my mind, the subtext became pretty obvious with the whole Grindelwald thing. It makes that whole arc of his backstory make a lot more sense (which it didn't when I thought of it as a purely platonic friendship), and adds some depth to his past - if she'd gone forward with it it explicitly would have been an interesting chance to look at her whole "love is the most powerful magic" thing from another, darker and more nuanced angle (ie. his love for Grindelwald allowed him to allow terrible things to happen, in the same way that Lily's love for Harry allowed wonderful things to happen). It would also explain why he'd avoided romantic involvement - not just because he had to keep it on the DL or anything because of prejudice, but because he'd seen that weakness in himself, that potential to get swept up by the power of love and allow it to become something terrible - in the same way that he said that he constantly refused the post of Minister for Magic because he feared what he would do with such overt political power.
So I think she definitely wrote him as gay, I doubt it's just something that popped into her head after the fact, her only real problem was saying "Dumbledore is gay" (paraphrasing) conclusively, when, however she wrote him, there's no definitive evidence in the novel. If she'd just said "well I initially conceived of him as gay" or "well I think of him as being gay," that would have left more room for people to have their own interpretations based solely on the written work.
