"The Atlantic" --- no hotbed of conservative gay-bashers --- has said as recently as October 2015 that science has not discovered as "gay gene." Do you have any better information?  No?  Well, then, scientist, since it's not scientifically proven that homosexuality is determined by birth, why doesn't your whole argument go poof?  

You also say  that because homosexuality is found in the animal kingdom, it is "natural," and therefore, morally permissible for humans.  But so what if some animals engage in homosexual acts?  They also commit bestiality.  (Oh, get a life.)  Absurd?  Of course, but since your argument is reducible to absurdity, don't blame me.  Point is, we humans don't gauge the morality of our sexual acts by the conduct of a (probable) minority of the animal kingdom.  If we did, then screwing at the zoo along with the giraffes and hyenas would be just fine.