The problem is genes regulate body form and not behavior. There’s no such thing as “predisposition” to behavior, except in the minds of liberal apologists that call alcoholism a disease and homosexuality an “orientation”. The practice of homosexuality is a paraphilia and should be seen as the pathology that it is. With your “logic” I guess there are “incest genes”, “bestiality genes” and “pedophile genes” too!
Disclaimer: All of the above is conjecture and hypothesis on my part, but it does seem to fit the anecdotal facts and observations.
Justifying perversion requires a lot of mental aerobics.
Sez you. Birds build nests. Kangaroos box. Infants suckle. You'll have to do better than that.