"If a person is attracted to people of the same gender, he or she cannot have biological children with their chosen partner." Simply not true. Many gay men either have children with a woman, or pay a surrogate to deliver their child for them. So gay genes could still be passed on in this way.
Regardless, I don't think people are 100 per cent straight or gay. I believe in a continuum, and men are usually mostly attracted to women and women are usually mostly attracted to men. Bisexuality is reality, even if most people are drawn mostly to the opposite sex.
So even though gay men and women don't just wake up one morning and choose to be gay, there is some measure of choice involved in mate selection. And there is NOTHING wrong with that.
As a gay man, it makes me uncomfortable when others gay try to insist that sexual orientation is entirely genetic. There is nothing wrong in choosing to be gay.