Hard to say, honestly. I know identical twins. One gay, one not. Have known essentially since birth and the one who is gay has always shown traits and characteristics of the other gender, even when we were 4. My educated guess is something happened en utero, after the split to twins, but very early on in development.
Have often thought that homosexuality was nature (God’s?) way of controlling the population a bit..ideally would just keep to themselves and live life. The ones I know, do just that.