IIRC, current research suggests a little of both.

There is a genetic component to homosexuality, but it can vary in effect. What it does, at a minimum, is create a *predisposition* to being gay.

Some people can ignore/fight that, or never encounter the environmental/social triggers for it, and live life as a heterosexual.

For others, however, they're simply gay. No gray area.

There's also been research showing that being sexually abused as a child can make one more likely to be gay. So a sufficiently traumatic experience can apparently reset one's sexual orientation.

__________________
Man up.