Here is what I do know from growing up around a few friends that did turn gay later.
I have noticed that some guys were simply "gay" or rather had a bit if "girlish-ness" about them all their lives.
I could see something odd about them from the start, and as we got older this became more and more clear to people that they were just not right from the start.
On the other hand....
I also knew a guy who was just a normal guy all the time I knew him in grade school and in high school.
There was nothing odd about them at any point, and they married and had kids.
Then....one day I learned that over the last year they had been involved with some gay guys and ended up walking out on the wife.
I was shocked and I never saw this coming.
So are some "born gay"?....I think there is a case to be made that for the people I have known all their lives, they sure showed signs of being gay from the start.
Others are clearly not born gay, and they have made a choice..