Zero evidence for it. Less than 10% are exclusively homosexual, until the fake ;science' campaign to BS the public with the 'born that way' scam. There is no 'gay gene'.
A massive study of half a million people finds no single gene behind sexual orientation, adding more evidence that there is no “gay gene”
www.scientificamerican.com
But collectively, genes do play a role in sexual orientation.
www.livescience.com
Best ongoing study on it by a biochemist.
condensed summary of My Genes Made Me Do It! - a comprehensive scientifically-based examination of homosexuality
mygenes.co.nz
Then there are all those men who were married for decades, had children, then suddenly became 'homosexual'.
Before the current brainwashing campaigns and fake science over 65% of homosexuals claimed to have been molested as children by adult males. Now these days they're told to parrot the BS that they have 'always felt homo n stuff' to further the 'normalization' scam.