The naturalistic fallacy, subtle style

Via PZ Myers: An adjunct professor at the University of Illinois Kenneth Howell has written an email to his students in which he defended the Catholic position on homosexuality. Here's an excerpt:

But the more significant problem has to do with the fact that the consent criterion is not related in any way to the NATURE of the act itself. This is where Natural Moral Law (NML) objects. NML says that Morality must be a response to REALITY. In other words, sexual acts are only appropriate for people who are complementary, not the same. How do we know this? By looking at REALITY. Men and women are complementary in their anatomy, physiology, and psychology. Men and women are not interchangeable. So, a moral sexual act has to be between persons that are fitted for that act. Consent is important but there is more than consent needed.

This reasoning is an instance of the naturalistic fallacy, albeit in a somewhat subtler form. In its naive version, the naturalistic fallacy states that what is natural is moral and vice versa. This is grotesquely absurd: rape and murder are natural, for example, so they would have to be adjudicated as moral. Howell doesn't go as far as to say that morality and naturalness are equivalent; he adds consent as another criterion for judging an act to be moral. More precisely he (seems to be) saying that consent is a necessary, but not sufficient, condition for an act to be qualified as moral. (IOW, all moral acts are consensual but not all consensual acts are moral.) To fulfill the morality definition, an act must be consensual and natural (as he says, "be a response to REALITY").

But this definition is absurd too. I'm sure any of my readers could come up with an indefinite number of examples of consensual acts that pretty much everyone on the planet agrees are not immoral but which nonetheless are highly unnatural. To give just one example: If he's consistent in his beliefs, Howell must think that flying in an airplane is immoral. It doesn't matter that people do it voluntarily; flying is not appropriate for people. How do we know this? By looking at REALITY. There is nothing in human anatomy or physiology that would suggest humans are meant to fly. So, a moral act of flying has to be left to species that are fitted for that act.