Your theory is flawed. Gay genes are probably recessive.
This means when you breed a guy and a girl together, assuming both
of them have the gay gene, the out come wll be something like
this.
By Punett squares:
S G
S
G
S-straight gene
G-gay gene
Outcome 1: SS (straight)
Outcome 2: SG (straight)
Outcome 3: SG (straight)
Outcome 4: GG (gay)
So gays will never be terminated.