Contrary to the assertion of conservatives that homosexuality is unnatural, an overwhelming body of evidence is growing which says that homosexuality is naturally occurring, even congenital. The science to support this conclusion is coming from psychiatry, neurobiology, endocrinology and a host of relevant animal studies. Already back in 1997 the Atlantic published an excellent article summarizing the variety of research which pointed toward the conclusion that homosexuality is biologically conditioned.
It is important to note that no one has identified a singular cause of homosexuality. There is, as of yet, no "gay gene," and even if a genetic link is found (which seems likely since various twin studies have determined that homosexuality is up to 70% heritable) it will only be a contributing factor, not a sole cause. No gene is sufficient to explain the diversity and complexity of human behavior. Most researchers agree that a combination of factors contribute toward homosexual attraction and behavior, ranging from genetics, to pre-natal hormone exposure, to brain structure.
Some conservatives complain that the science is politicized and inaccurate. One can never rule out politics and bias in anything, but the studies are so numerous and the results so compelling that regardless of any bias in reporting the conclusion is persuasive to anyone paying attention. The most common complaint of conservatives is that the American Psychiatric Association since removing homosexuality from the DSM in 1973 has ignored the proponents of reparative therapy. However, this is not a case of political bias, but rather the overcoming of an unreasonable bias. For over 100 years it was the completely baseless assumption of psychiatric medicine that homosexuality was a pathology. The position of the APA was only reversed after study after study had failed to prove homosexuality was either pathological or curable. Indeed, not one clinical study supports the idea that homosexuality is a disease. In anonymous survey after anonymous survey psychiatrists have failed to even identify sexual orientation based on psychological profiles. The APA correctly warns against reparative therapy because it is based on century old bad science and treats homosexuality as a disease which it is not.
The fact that homosexuality is neither a disease nor a simple choice, but a biologically conditioned identity accompanied by feelings and behaviors is relevant to the question of whether or not homosexuality is a sin because it destroys Natural Law arguments. Homosexuality is natural, therefore we cannot call it a sin on the basis of it being "unnatural." That line of argument is terminated. Good riddance.
The implications of homosexuality being natural reach also into that other demense of conservative moral reasoning - divine command, because wherever a "divine command" can be demonstrated to rely for its justification on a mistaken understanding of human nature we are right to point out the flaw. That is, all of the assumptions various authors of the Bible made about homosexuality being unnatural are revealed to be wrong, just as parts of the Bible are wrong about the sky being a metal dome or rabbits being ruminants. If the rationale for the rule is wrong the rule is also wrong.
Ironically, though, progressives often make the same mistake as conservatives in reverse. That is, if conservatives say "unnatural = bad," progressives often assume "natural = good." This is not the case. As much as biology can help us to understand how we are made, and where some of our feelings and behaviors begin, it does not have the final say about the moral nature of our actions.
It is possible, for example, that some people have a genetic predisposition toward violence, but that would not make violence acceptable. It may, however, influence our judgment of the person who committed the violence if they were genetically so predisposed. We might lean toward treatment or containment rather than punishment in our response. A biological condition cannot by itself make an action good or evil, but it does play a role in our calculation of moral responsibility.
Thus we can make a few modest conclusions on the basis of the knowledge that homosexuality is biologically conditioned. First of all, we have discredited any argument which relies on the assumption that homosexuality is unnatural. However, we cannot leap from there to the positive assertion that because homosexuality is natural it is also good. Regarding homosexuality as natural does influence our calculation of moral responsibility, nevertheless, to make a decision about the moral nature of homosexuality will require something more than all of this obsession with nature. Deontology is not enough. We need to examine intentions, circumstances and consequences.