Galt is not arguing that homosexuality is exclusively genetic or developmental, but only that homosexual advocates for heredity may regret it if they get what they wish.Depends on the potential parents. And that's where Science would have a problem IF it wants to continue the Born Gay DNA Argument and Research.
The "Born Gay" theory/hypothesis was put forth by a Homosexual Scientist. Not exactly unbiased. And certainly not hard Science or Scientific Fact, at present.
If Science did isolate the "homosexual gene", potential parents wouldn't necessarily have to pass on reproducing or opt for gene therapy to eliminate homosexuality.
They could just terminate the pregnancy through abortion.
PS Ever see the movie "Gattaca" ?( 1997 ~ With Ethan Hawke, Jude Law, Uma Thurman ).
As an aside, how could any male child of lily or Jean from Queens possibly grow up to be heterosexual?