Many studies have been carried out to try to understand why some people have same sex orientation and the conclusion (in the overwhelming majority of cases) is that we are born that way.
It can't be a choice. You can't choose what and how you feel. The only thing you can choose is how to live your life, but even that sometimes is impossible
From thinking I'm straight to falling out of control into a woman, I can say it's certainly not a choice!
There's overwhelming scientific evidence that sexual orientation is a result of hormone exposure as an embryo and other variables. There's also some strange and interesting correlations, such as proportional finger length and the direction your hair whorls. If you google "The Science of Gay," the first link is a neat infographic that explains it all nicely.