This topic comes up time and time again.
Unless it's proven that there is a gay gene or brain I cannot believe that people are born gay. I'm a skeptic and I need hard proof to believe something. The Flat Earth Society still believe the earth is flat. Some people believe aliens visited Roswell, NM in the 1940's. And there still are people who don't believe we landed on the moon. I believe the earth is round, I don't believe aliens visited Roswell, and I do believe that we landed on the moon. I'm also not convinced of global warming.
I think our sexuality is a combination of many things that stem from our childhood; how we were raised, our relationship with our parents, early sexual experiences, interactions with our peers, our schooling, and many, many things. But I also think it's very complex and we may never know or even understand the complexity of it. Some people consider themselves gay or straight based on who they have sex with but others don't. That in itself is confusing.
For example, this thread about a straight man trying gay sex. Personally, I thought it was fiction, but you never hear it the other way around.
http://www.lpsg.org/56983-my-first-gay-experience-straight-5.html
Where are all the gay men who have had gay sex their entire life and then try sex with a woman and say "That was awesome. No more men for me!" You don't. The straight man in that thread chose to have gay sex, and no that does not make him gay, but yet some will say that having even just one same sex experience makes one gay. Other people will say that our actions do not define our sexuality, so right there is a difference of opinion.
I was talking to this gay man one time, and he said he realized he was gay when he was 16. When I asked him how he knew it, he said something like(I'm paraphrasing) "I was always into women, but I couldn't get a girlfriend like my friends could. Women didn't find me attractive. So I started hanging with another group of guys who were gay and we started fooling around. I started enjoying it and after that I never bothered with women again." That sounds like a choice to me. It sounds to me that if this man had found a woman then he would have gone straight.
Obviously that's just one case, but it's interesting because he figured that at 16 if he couldn't get a girl so he couldn't be straight. Then he got guys to like him so he was gay.