It depends on how you're looking at homosexuality; are you looking at it as sexual attraction, or as behaviour? If you're talking about behaviour, then a lot of that is certainly environmental, e.g., if you're raise a non-permissive location, you're much, much less likely to engage in homosexual behaviour. But if you're talking about sexual attraction, then it seems very unlikely that it could be anything other than primarily genetic.

I think that the fact that there's a difference between how people act, versus how people feel, is what confuses so many people about being straight, gay, bisexual, transgender, etc., and why conservatives feel like there's a 'gay agenda' to make kids gay (or trans) when a permissive society allows more people to act freely on the way that they feel.