There is evidence. See the fraternal birth order effect for just one example:

https://en.m.wikipedia.org/wiki/Fraternal_birth_order_and_male_sexual_orientation

And scientists have created lesbian mice:

https://bmcgenomdata.biomedcentral.com/articles/10.1186/1471-2156-11-62

(Side note: it's hilarious they shorthanded the enzymes to FucM)

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.

Yeah, I think you are right and I couldn't find anything. I just find it highly suspicious that my dad is hardcore homophobic when more than half of his immediate family is gay or bisexual. Like, even his brother is gay.