What counts as a cause is certainly a philosophical question, and although both laypeople and scientists often confidently talk about causes, philosophers are far from unanimous about the correct analysis of the concept of causation. It's a matter of considerable controversy: see, for example, this SEP entry.
But if we achieved a precise enough and plausible enough understanding of cause and a precise enough and plausible enough understanding of the concept homosexual (perhaps easier but by no means easy), then I think the question of what causes homosexuality would be purely empirical rather than philosophical. It may not be a purely biological question (again, assuming we understand biological precisely enough), but I don't think it would be, at that stage, a philosophical question. Still, lots of philosophical work would need to be done before the question became well-enough understood to be answerable empirically. Or so it seems to me.