I would never use this opportunity to interfere. It's not my call and I don't think I have the right to do it. I would let nature take its course.
Having a homosexual or lesbian kid is a challenge especially because of people who think they have the right to judge others for being different but that doesn't mean we have to interfere. Society needs to change, people need to accept reality and deal with people being different.
Of course I would NOT predetermine such a thing. The thought alone is utterly preposterous. Only homophobic people can come up with such a plan. Homosexuality is no disease, that would require treatment or prevention. Homosexuals do not need to change but what must change is society. I sincerely hope that such genetic manipulations will never become reality.