Being gay is no more a "genetic disorder" than being born with blue eyes, or blonde hair.

It's a hidden genetic trait that aids in population control (imagine how many ppl there'd be in the world today if all homosexuals in history had reproduced), and is merely one in a countless ways our species, and many others species, keep up it's variety.

Natural selection doesn't always favor the "correct" method. It tries many different ways to keep its species alive. A mistake or not, the first homosexual animal was only one variation, probably of many, of nature's plan to keep us thriving. Just because it doesn't aid in reproduction does not mean it did not in some way help a species thrive. Perhaps primitive homosexual animals posessed some advantagous trait that heterosexual animals did not. Perhaps that was the trait that lended the strength to the pack in order to keep it going. Maybe a very early species needed homosexual behavior because that's what the female portion of the species responded to. It could have been a wild mating call gone awry.

A trait can get passed on regardless of whether nature wants it to or not. A bunch of gay animals were clearly still able to reproduce, passing this probably extremely recessive gene onward.

It's anything but a choice.

I was born gay, and I'll die gay. Discovering you are gay is a whole different ballgame. Just because you "find out" you're gay doesn't mean that's when you suddenly become gay.

All of the sudden turning gay is as likely as waking up to find you've had a genetic nose-job in your sleep: impossible.

edit: i'm not sure if having the capability to choose your childs sexuality is any more or less ethical than being able to choose your childs eye or hair color (something that is far more likely to happen before choosing sexuality). It will still affect the child's life somehow.