What if some prospective parents are conceiving a child, and the doctor comes up and says, we've determined by your child's genetic structure that he's going to be Gay, but we can fix it, your child can still be born, but he will not be gay if you like. What do you think the child's parents are going to do?
Funny that your premise incorporates the notion that being gay is something that needs to be fixed. The fact that you chose to frame things that way makes your question at the end there seem rhetorical.
I don't believe that 24th century doctors will, as a rule anyway, be thinking of homosexuality as a condition that needs to be fixed. I don't believe that people in general will be thinking of homosexuality as a condition that needs to be fixed either.
Therefore, my answer is, to the literal text of your question: the parents will probably immediately seek a second opinion. The next doctor will probably simply tell them that their child seems perfectly healthy and there's nothing to worry about; homosexuality is not a condition which endangers the health of the individual or others, it's not even a medical condition at all, and incidentally nor is it anything that even remotely endangers the survivability of the species or the health of possible descendants (yes, gay people have children, too).
That being said, no doubt
some parents and
some doctors would favor genetic modification, à la Chakotay. (Of course, the premises that genes alone or at all cause homosexuality in the first place are problematic hypotheses, for numerous reasons, but for the sake of argument let's leave that aside.) The fact that some parents and doctors are this way might make for interesting stories, e.g. about brutality in utopia. As I said upthread, homophobia will probably still exist in the future, to some degree.