Are people born a-sexual or are they born gay or straight?

Are people born with a sexual preference? Is sexual preference decided? Is it molded by many different factors you've endured through out your life?

I personally believe sexuality is not a choice and it is not genetic. I think people are a-sexual and various life experiences and environmental factors make them gay or straight, but I would really like the opinions of other people. Perhaps people who actually know what they're talking about-unlike me.