Ask your friend if he/she believes in evolution.
Then say that all of the gay animal populations became extinct and that is why there are no populations of gay animals. After all, it is survival of the fittest and obviously the gay animals were not fit to survive.
Why would humans be the only mammal with this so called gay gene?
Before you read further, understand that I do not believe that homosexuality is genetic. Rather, I believe it to be a developmental issue. Whether it’s purely psychological or physiological, I cannot say. However, bad science bothers me, so this needs to be said.
To say that homosexuality cannot be genetic because it would die out with no direct transmission is faulty logic, that belies any understanding of recessive vs dominant genes.
Example: Using your logic of reproduction, there would be no muscular dystrophy of the Duchenne variety, as the afflicted rarely live to an age to directly pass on the disease. It would just die out with the afflicted. However the disease is genetic, it persists and it’s unlikely to go away. The reason is simple, unaffected females regularly pass the gene on from generation to generation.