I don't think being gay is a choice. What you do about it is. You can choose whether you want to get into a homosexual relation or not. But as others have pointed out, I couldn't just choose to be attracted to men, I'm attracted to women. I'm assuming that a gay person is the same way.
There may be some part of homosexuality that is born into us. Hormone balances and the like, but mostly I think it's probably due to early life experiences. And it's probably not obvious things like being exposed to the idea of homosexuality at an early age or playing with Barbie dolls or any of the things people commonly suggest.
I heard once, from an UNreliable source, that there was a study that found a link between exposure to pornography at an inappropriately early age and growing up in a household where the mother was the dominate parent, and homosexuality. I've never been able to confirm the existence of this study. If there was such a link it would help explain the rise in homosexuality, with cable TV and the internet making it easier to access porn, and with more single parent households. It could just be that the wider acceptance means that more people "come out" rather than hide it or live a lie.
One thing I object to is the mainstreaming of homosexuality. It's obvious to me that regardless if we are the invention of some god or the result of evolution that the design intent is for males and females to have sex with each other and produce offspring. That to me is the "normal". Anything else is a deviation from the normal. I think were being dishonest with ourselves when we deny that just to be PC. It's because of this denial that there is little research into why some people are gay.
As to the gay marriage issue I think the government should get out of the business of defining "marriage". First, I think we should give the word "marriage" back to the society i.e. churches, religions, and people in general, and let them/us decide what it means. Next, we should look at our laws and ask the question, Why do we have laws that treat 2 people that have decided to unite together differently than any other 2 people in society. The answers I can come up with are: to make it easier for them to care for each other; To make it easier for them to plan a life together; and to make it easier for them to raise children together. There may be others that I have not thought of. Once we figure that out we can rewrite our laws so that they go to benefit the people intended regardless of gender or religion.