If there is a gene for homosexuality,
there is not a homosexual gene. There are biological factors that affect sexual orientation. Current research suggests hormonal influences in the womb are a big factor.
but sexual orientation is not a 1/0, yes/no, gay/straight type of thing. It is a range, let's say 0 to 6. and your sexual orientation is somewhere in that spectrum. If you are a 0 or 1, it would be very hard for you to fall in love with or have sex with someone of the same sex. If you are a 5 or 6, you'd find it impossible to do those things with someone of the opposite sex. But if you are in the middle, you could go either way. You could be truly bisexual, dating both men and women regularly. Or you could spend years and years thinking of yourself as either gay or straight, then suddenly deciding to live the opposite way. While your behavior might have changed, your orientation did not.
While we're all born with a sexual orientation, those of us at one end of the spectrum or the other can choose to live by our orientation, or we can choose to be celibate and alone. I see no reason to do the later.