If you look back to more ancient civilisations like the Greeks, you'd note that homosexuality was widely accepted at the time. But then, I believe, as the Catholic church later gained power, it was suppressed and condemned, so now society's ideals as a whole are anti-gay. I believe there have always been gay people, the problem is they've just been too scared or too intimidated to be able to come out and admit it. (Note: I'm just going by what I remember here...haven't actually read the facts in a few years so they're probably a little off or something. :x)
As for the born with it thing, well...I still don't believe people choose it because honestly: why would someone choose to be that which society condemns? (The conditions are improving, so I can see some people doing it for attention--I think I know someone claiming to be bi just so that people will think she's cool. <_<) Part of it could be determined just how someone's raised. But then, even that has its decrepancies since people who come from strict Christian families can turn out to be gay too...and growing up as they did, it doesn't seem to make sense. o_O It could be a combination of nature and nurtute, but I truly don't believe that it's purely based on choice. But then, this is kind of a whole other debate, isn't it? XD