Jesse Bering: The Naturalistic & the Procreation Fallacies.

When religious or social conservatives commit the naturalistic fallacy, [...], the issue of procreation usually takes center stage and the philosophical error is embarrassingly salient. The trouble with confusing morality with reproduction applies equally to any sex act that can't reproduce offspring, but it's most commonly seen, unsurprisingly, in arguments against homosexuality. Many social conservatives enjoy pointing out to those of us who just can't seem to grasp these more challenging aspects of biology that whereas having intercourse with the opposite sex can produce offspring, having intercourse with the same sex cannot. This is usually extrapolated to mean that gay sex is obviously unnatural, and so it's obviously just plain wrong (translation: arrogantly ignores God's intentional design.)

Given that nature is mechanistic and amoral, and not the product of intelligent forethought, this entire position is a nonstarter. To draw from assembly-line principles of reproductive biology any moral directive or prescription for what human beings should and should not do with their genitalia is to assume that a Creator intentionally engineered our reproductive anatomies. That pre-Darwinian view--that a preconceiving mind is needed to account for the evolution of our body parts and that this divinely penned blueprint should in turn dictate our social behaviors--lies at the malformed heart of the religious arguments against homosexuality (one that's usually expressed in the form of some awful rendition of the "Adam and Steve" fundamentalist refrain).

We've become so focused as a society on the question of whether a given sexual behavior is evolutionarily "natural" or "unnatural" that we've lost sight of the more important question: Is it harmful?

Bering, Jesse (2013) "PERV" [pp.20--21]