As a gay man I'd say it is 100% not a choice. Who would choose it? When you're in school it is terrifying of being outed by anyone. You think your parents/mates will reject you if you dare come out. You're almost living half a life until you tell that first person.
I told my best mate nearly 2 years ago now and my parents last year when I was 21. After a rocky start with my parents it is now just not an issue.
Although I am gay, I have been with a lot more girls than what I have men. Even when I go out now, I tend to go with girls. I don't know why I do it, it could be an arrogance thing. In part though, I feel that I am doing the right thing if I go with women. After a man I'd go through terrible regret for a long time.
There are more 'straight' men who have been with men than what you'd think.