I'm not sure if homosexuality is the result of genetics in such a direct way, but if it was, there is a kind of genes called recessive genes. So if they were to have the gene, it might be dormant for a generation, or the majority, but it occasionally shows up. Again, I'm not sure if its exactly gene related, and i dont know much about genetics so if i got the terminology wrong i apologize, but that should answer your question.
On another note, i dont feel like typing out a long winded response to everything in this thread, but I'll just say that on this particular subject, i fully agree with RubberSoul on 99.9% of what hes said.