Wikipedia on causes of homosexuality says there are "various biological causes", and quotes:

I don't much care about quotes from very prestigious professional bodies; they are always mealy-mouthed and self-serving, where they do not endorse pleasing claims on radically insufficient evidence (eg diet). What's more relevant is the actual research, like the link to https://en.wikipedia.org/wiki/Biology_and_sexual_orientation ; I'm not too impressed, as it seems to be mostly a laundry list of fairly dubious corrrelations and downstream effects or irrelevant to humans animal research (does anyone think that deleting an entire gene in mice to turn them gay tells us anything relevant about causes in healthy humans?).