I think the argument against changing things non-medical is that to do so you have to make a decision that is clearly not yours to make. For instance, in an abortion there is no eventuality that leads to an adult, thus it's supposedly OK. The difference being that in this genetic manipulation the child is ALWAYS meant to live, so doing anything cosmetic to it (even while it's young enough to abort) is unethical, seeing as how it would have to live with the changes you so kindly chose.
It's the same kind of thing as slicing an arm off while the baby is < 6wks because you like the look better and then bringing it to term. IDK about you guys but that seems rather unethical to me, regardless of whether you think the baby was a "person" at the time of the slicing. I don't think such subtle distinctions would matter much to the dude with one arm.
Something interesting I noticed about about altering the gay gene is that its supporters must actually acknowledge that there IS a gay gene... Heh.