There are two lines of thought here: 1) Sexuality is a choice. 2) Sexuality is genetic. With either of these concepts you can not claim that heterosexuality is natural. If sexuality is a choice then your sexuality is learnt; and if your sexuality is genetically heterosexual homosexuality can not exist. Conclusion: Your argument is not valid.

You're working from the assumption here that all humans are born heterosexual. There is no evidence to suggest this. So if your platform is flawed, your conclusions will be flawed.

Most scientists these days agree that sexuality is probably a result of a mixture of nature and nurture- meaning that both genetics and your experiences in life both play a role in determining whether you will be homosexual, heterosexual, or bisexual.

As there is no scientific evidence that supports the theory of a biological influence on sexual orientation, the statement of humans being born either homosexual or heterosexual is without merit. Although many have stated that since humans are the only animals that display homosexual tendencies and thus cannot be born homosexual, recent studies have shown that there are many other species that also display homosexual tendencies. Some examples are certain varieties of beetles, birds, bats, sheep, dolphins and oragutans. Scientists are learning that bisexuality and homosexuality, once thought of as nonexistent in the animal kindgom, is actually quite prevalent.

Time for another edition of Everything You Know Is Wrong: 1) Heterosexuality/Homosexuality is a false dichotomy. Instead, most people are somewhere on a continuum somewhere between the two extremes. 2) The argument that homosexuality cannot be genetic is based on a very simplistic understanding of genetics. Not all genes express themselves in every individual. Furthermore, many animals (and humans) display altruistic behaviors that are not in their immediate best interest for propogating. The current understanding is that these behaviors benefit the sacrificed by allowing their close kin to survive and pass on a large majority of the same genes. Under this logic, it is possible that human hunter-gatherer tribes that had 5%-10% homosexuals in their population may have had a survival advantage over those that did not, because those individuals did not participate in the struggle for mates or have children to raise. Hence, those indivdiuals spent more effort focusing on the survival of the tribe as a whole. 3) Homosexual behavior has been observed in numerous animal species. So, unless you are willing to argue that apes, dolphins, swans, penguins, and other creatures have the same capacity for 'choice' as humans do, there clearly is an instinctual (= genetic) component to homosexuality. 4) Our general concept of "homosexuality" is largely a modern construct. If you look deeply into history, you will discover that men and women have engaged in all sorts of same-sex bondings in all sorts of situations. For example, some of author Jane Austen's personal letters strongly indicate she had quasi-lesbian relationships with her sisters and friends. Similary, there is documented evidence that men in remote places (like mining towns) where few women were available turned to each other for satisfaction. Even Abraham Lincoln is suspected of having a 'gay' companion. History indicates they didn't think of this as homosexuality. They thought of it as getting by until a situation where the other gender was more available came about. This idea of 'homosexuals' as a group identity emerged largely during the Victorian era. 5) Finally, think to yourself...when did you "choose" to become sexual (hetero-, homo-, or whatever)? Did you wake up one day and decide "Gee, I'm going to be sexually interested in girls (or boys) today"? Nonsense! Your sexuality was triggered in your brain by a complex combination of hormones, neurotransmitters and cellular development. You most likely can't point to an exact time/place when it occurred. To assert that this complex process could not possibly have an alternate outcome 10% or so of the time is akin to asserting that left-handed people (about 8%-15% of populations) must have 'chosen' to be 'that way'.