A common defense for homosexuality is that same-sex attraction is natural for homosexuals. There’s nothing wrong with homosexuality - the animal kingdom is filled with examples of same-sex erotic behavior. It's natural.
Now on the surface, there seems to be some truth to the claim that homosexuality is natural. It’s not unusual, for example, to see male dogs mount each other in an erotic way. There are two problems with this view, however.
First, the observation is flawed because it assumes that erotic behavior in other mammals is the same as homosexual desire in human beings. Male homosexuals engage in sodomy because of an attraction to a gender. They are male erotic, and sodomy is an expression of that desire.
Does the animal kingdom display this kind of same-gender eroticism? When a male dog mounts another male dog, is it because he’s attracted to the male gender of the other dog? I don’t think so. This same poor pooch will slavishly mount sofas or shrubs or anything else available, including the leg of your dinner guest. None of these things are the object of the canine’s sexual lust; they are merely the subject of it. The dog does not desire your unfortunate visitor. He simply desires to be stimulated. It doesn’t prove they have same-sex attraction that is in any way parallel with humans.
Secondly, the view asserts that simply because the condition “occurs in nature” it is “natural.” But by this use of the word all sorts of things would be natural—humans mating with animals, children drinking cleaning fluid, rain forests being replaced with concrete—because all would be occurring “in nature.” Human beings are part of nature by this definition, and therefore all and any human conduct would be natural. Virtually nothing could ever be considered unnatural on these terms.
Generally we mean something else when we say that replacing virgin forests with parking lots, imbibing poison, and copulating with beasts are not natural. Things are natural if they fit the pre-technological, natural order of things; they are functioning according to their primitive pattern or purpose, and that’s key. A natural sexual desire, then, is a desire that serves to accomplish a sexual goal in the primitive order of things.
However, this definition of “natural” doesn’t help the homosexual either. According to the primitive natural order of things, the natural purpose of sex is reproduction, getting one’s genes into the next generation. But homosexuals don’t reproduce, so homosexuality can’t be “natural” on this definition either.
Further, wouldn’t one consider it unnatural if someone had eyes, but couldn’t see, or ears and couldn’t hear? Why is it then deemed natural to have male genitals, but be dispositionally incapable of using them to accomplish their reproductive purpose with the opposite sex?
Homosexuality does not seem to be natural in any meaningful sense of the word. Therefore, its morality cannot be defended by a mere appeal to nature.