Ray wrote: The fact that something exists suggests that the certain something exists in nature. "Natural" or "unnatural" arguments are vague.
In the context of this discussion, naturalness is defined as having a genetic cause. Sexuality most likely has an underlying genetic cause because that is how we, as an animal species, reproduce. But as genes can not determine the gender of a sexual partner, only initiate and probably regulate the desire to behave sexually, the chosen partner is most probably a culturally determined decision. This does not mean that any sexual orientation is valid or invalid. That validity, or if you will the legitimacy, of the attraction of one individual to another is also a cultural decision. If homosexuality were simply a genetically based behavior, it would have disappeared long ago because the activity is not reproductive. It is still here, quite healthy, and those with that orientation feel quite comfortable with themselves. Which is perhaps the strongest argument for a non genetic basis.
We should be very leery about claiming a genetic cause for any behavior, for this is the favorite argument of racists.