I don’t think that a question is off-topic simply because it discusses or mentions sexuality.
It still has to be within the realm of sci-fi and fantasy, but the two topics certainly aren’t mutually exclusive.
As for this particular question, I think what may have irked people was this line:
I can't see how this isn't a gay reference
It sounds a bit like you have a bit of an agenda asking the question, something like “This is a gay reference, isn’t it? Prove that GRRM is lying”, which isn’t constructive or useful. That may not be your intention, but it’s how it could come across.
A better question might ask why the Rainbow Guard is named as such in the books. You could mention the fact that you think it might be a gay reference, but it doesn’t need to dominate the entire question.