There are plenty of people already who have already posted the arguments suggesting that homosexuality is natural, so I'm going to get to a different root of the problem.
In rational argument, an appeal to nature does not generally follow. Just because A is natural does not mean A is good or right. Just because B is unnatural does not mean B is bad or wrong. It is one thing to determine that something is natural or not (say, by seeking out examples of a behavior in zoological observation) , but to base acceptance of that behavior as right or wrong based on its status as natural or unnatural requires further logical steps. It just doesn't follow automatically.
This is part of a larger issue of logic, the Is-Ought Problem [http://en.wikipedia.org/wiki/Is-ought_problem] which addresses the confusion that occurs between descriptive statements and prescriptive statements. Many fallacies of logic come under the presumption that certain characteristics are favorable (e.g. natural, traditional, avant garde, commonplace, normative, modern, intentional, etc.) when discerning what should be on moral or ethical grounds.[footnote]Think about this when a given advert uses these kinds of descriptors as to why you should buy this product, or our government representatives who will use such terms to bolster their own position, or dismiss those of their opponents.[/footnote]
One example comes from a development in the psychiatric sector, regarding people who hear voices. Traditionally, hearing hallucinatory voices that no one else could here was regarded as a symptom of schizophrenia, and was treated with lithium or Thorazine to make the voices cease.[footnote]With accompanying terrible side effects. While we have better drugs these days, they still are pretty severe, and it's more a matter of choosing lesser effects, or at worst less inconvenient side effects.[/footnote] But then one had to address, what of those who hear the voice of God? some of whom were (or are) spiritual leaders of thousands. Are we to say they are crazy too?
In the 1990s a study was done comparing the experiences of those who just heard non-particular voices, and those who heard the voice of God.[footnote]This study intentionally left out those who heard particular, but non-divine voices, such as a dead relative.[/footnote] The voices of God were significantly more benign to those who could hear them compared to those who heard the other voices, and patients who heard God were usually disinclined towards treatment to make them go away.
This caused quite a bit of disruption in the way we regarded psychiatric treatment, not only of schizophrenics who suffered from hallucinations, but of anyone diagnosed with a mental condition. The question was raised Who are we to judge what is a disorder, and what is simply another state of mind?, and it is from this consideration that we developed the current standards for practice, specifically that in order for a symptom to be regarded as a disorder, it has to have some kind of dysfunctional effect in the patient's life, and the patient decides if, for example, it's worth it to continue hearing God, if the voice is too loud to allow him to concentrate on driving a motor vehicle.
Similarly, in the courts of law in the US that so far have addressed the issue of homosexuality (and numerous other fringe practices), it has usually been established that before a practice is proscribed or restricted, it is a burden of the court to establish proof of harm caused by the practice or behavior in question. So, before laws can be passed, for example, restricting the sale of violent video games to minors, the proponents of the bill must establish to the court's satisfaction that there is sufficient evidence that violent videogames played by minors harms the players, or brings harm to the community. So it is with homosexuality. And most courts so far have recognized that ours is a diverse and polyfaceted society, and we can't be so quick to pass laws that needlessly infringe upon our personal liberties.
238U.