There is a fallacy of logic that is so common that few people realize that it is, indeed, a fallacy of logic: The Fallacy by appeal to nature.
One argument against homosexuality goes something like this:
The natural use of the reproductive organs is obviously to make children. Using these organs in a way that cannot produce children is unnatural and thus ethically wrong.
There are many lines of argument against this argument: 1) pointing to species like the Bonobo, where homosexuality is common, and what occurs in nature without human interference is by definition "natural". 2) Many "uses" of various limbs is obviously contrary to "natural" use, like using feet for gas pedals in cars, or fingers for typing. Also, that line of argument would force you to argue against birth control and masturbation and even, by extension, use of medicine against the very natural diseases.
The real fallacy of the argument, however, is one originally pointed out by the philosopher Immanuel Kant in his theories of ethics: You cannot go from an is to an ought. The way something is cannot be a logically sound argument for saying how it ought to be. It is a confusion of descriptive and normative.
The reason we accept this fallacy so easily is that we're brought up to think that "natural = good". How many products aren't advertised with a reference to their "natural ingredients"? This romantic, Rosseauan view of nature is not very consistent with reality. Nature's "natural" inclination is to try to kill you with disease, an unhospitable climate and starvation. In our modern society the struggle against nature -- what keeps us alive -- is so mechanized and effective that we take it for granted, and we receive romantic nature photographs through the television. Poisons are just as likely to be natural as chemical or artifical products. If we think through these issues, we will realize that politicians, environmentalists and marketers keep misusing this fallacy and misunderstanding.
The flip-side of the anti-gay fallacious argument above is to say that because homosexuality exists in nature, then it is ethically good. That does not follow. Murder, incest and cannibalism exist in nature, but should not be used as a basis for human morality. Another common pro-gay argument is the below:
Evidence suggests that "gayness" may be caused by a gene. That would mean people are born gay. Therefor, it is ethically good (or, at least, acceptable) to be gay.
Considering that both gays and Christians alike seem to accept this argument, it may surpirse many that it is simply a variant of the above fallacy. Ethically, it makes no difference whether genes influence our sexual orientation or not. Even geneticists like Richard Lewontin and evolutionary theoriests like Richard Dawkins argue strongly against such gene-oriented argumentation, especially in ethics (a side note: while it's virtually certain that genes influence sexual orientation, it is very unlikely to exist a so-called "gay gene.").
Of course, it could be asked: if we had "a gene for" being violent, would that justify violent behaviour? The problem is, of course, that we obviously have genes for being violent. Human beings, especially males, are naturally violent. We must unlearn violence, as every parent will know. And the rules of society are made up to control our urges to be violent.
It could also be made a good case for inclination towards pedophilia being genetically influenced. A trend in human evolution has been that adult individuals look more like young individuals in humans than it did in our ancestors, and than it does in our closest relatives. So when men consider a woman pretty, she typically has 'childish' facial traits (term "baby face" comes to mind). But will this in any way justify those who have a sex drive "run amok" and directed towards children to live out such traits? Obviously not!
The "argument from nature" is fallacious when it used against homosexuality, but it is no less so when used as an argument the other way.
As an endnote, I have to point out that gays have no "burden of justification" to argue that homosexuality is OK. Those who argue that homosexuality is unethical have the burden of proof; an obligation to provide rationally sound arguments to demonstrate it is ethically wrong.
See http://www.infidels.org/news/atheism/logic.html#natural for more on the "The Natural Law fallacy / Appeal to Nature"
- Jan
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Faith, n. Belief without evidence in what is told by one who speaks without knowledge, of things without parallel. [Ambrose Bierce, The Devil´s Dictionary, 1911]