Oh wow, I can actually contribute to a thread and not feel like I'm talking out my ass!
I'd say that homosexuality probably has a grounding in a person's genes but is very strongly influenced by the influence of society, family, ect. You could easily have people who find men attractive but vigorously deny that they do. I know at least one person like that, and he never considered for a minute that he could be anything other than straight because he was raised in a conservative Christian family.
In my case I think it probably started with genetics, in that I am attracted to features of both sexes and probably would be even if I decided to ignore it and 'be straight' and that all got jumbled by the onset of puberty and the fact that I've never seen homosexuality as something wrong because both my parents drilled the fact that they're just people very hard into my skull from the minute I knew that homosexuality was a thing. Also it probably has something to do with the fact that I am very much a 'middle ground' sort of person and I hate having to choose.
So really, I think that people probably have a genetic predisposition towards what sex they find attractive (and I've heard a lot of people take kaijyuu's position and consider themselves straight but don't deny that they've found at least one person of the same sex attractive at one point) and sometimes that predisposition is so strong they can't ignore it, but at the end of the day it comes down to the choice to act on those feelings, a choice that is heavily influenced by factors like society, friends and family. So it could be mainly genetic, if that predisposition (which is a word I love and will use till it wears out and loses all meaning) is strong enough, which would lead somebody to believe they never had a choice, they were born this way, or it could be just somebody embracing their attraction to certain people of the same sex, which would mean they'd assume they did have a choice. Homosexuals (and bisexauls and pansexuals ect ect) are people, and people are a pretty diverse lot.
So this is one thing I really think homosexuals need to clarify within their own community, to the rest of the world.
And that's why I don't think they can. There isn't one definitive cause for homosexuality, or at least I seriously doubt there is and I don't think the community would ever be able to take a unified stand on the issue without alienating a lot of people.
One slightly off topic thing I wanted to ask, because I feel bad about it. Does anyone else find flamboyant homosexuals really, really annoying? I know it's awful of me and unfair on them but holy crap they annoy the hell out of me. Does anyone else (who is, you know, gay, bi, ect.) feel like that? Because I don't want to be both into dudes and homophobic.