This argument is based on natural selection, the fitness cost of genes 'for' homosexuality being too great for its occurrence at a frequency above that of random mutation (~ 1 in 50,000).

What if the allele for homosexuality is recessive (you need two copies of it to be homosexual), and having one copy of it has some advantage (the way having one copy of the sickle-cell allele makes you resistant to malaria)?