There is another logical argument you could use if your friend is open to statistics:
Genes are passed from one generation to the next via sexual reproduction.
If one parent carries a gene, but the other doesn’t, there is only a 50% chance the child will be passed the gene. Not a problem if gays reproduced at the same rate as non-gays, but they don’t.
Those carrying a gay gene would reproduce at smaller numbers than the rest of the population (due to a high percentage of them never engaging in reproductive intercourse, and thus never having children.)
Consquently, the number of people born with the “gay gene” would go down every generation until eventually it was eliminated.
So, even if there was a “gay gene” somewhere far back in our history, it would have been eliminated by now.
No. Again, I can point to many genetic diseases which completely preclude procreation, yet they continue to exist.
Your argument is based on a simplistic, grade school model of genetics that only deals with dominant allele. Even a jr. high understanding of recessives would be enough to shoot holes in the reproduction theory, let alone advanced genetics dealing with homozygous, heterozygous, hemizygous, and nullizygous combinations across multiple genes.
Politics is one thing, but politics should never lead to bad science.