Much as we don’t like the idea, sexual selection is a key strategy of sexually reproducing species. That’s why the sexes select for genetic soundness, evidenced by strength, competence, beauty, and all the other “norms” that the less well-endowed are “less than.” So the stream is going to flow against those who don’t look like the best reproductive partners. Indeed, the unattractive human almost certainly invokes a response akin to the uncanny valley effect of androids. We see such a person, and we fear ever so slightly for the fate of our children.

I’m not taking a moral stance, but I am saying that the ability and choice to make ourselves appear genetically fit are themselves traits for which mates may select. Self-discipline is a trait, as is the metabolism to be ideally proportioned if one chooses. In short, everything we appear to be is evidence of how well our offspring will fare, which is, of course, the primary drive of sexual selection. If you don’t care how you look, your kids may inherit your indifference, and that may prejudice their life chances, including their reproductive chances. Potential mates sense that risk through the mechanism of attraction.

It won’t do to trot out the old “not necessarily” ploy. Natural selection plays the percentages. You are not obliged to marry a visibly fit mate, and you may for whatever reason choose not to. In some cultures, fitness may look different than in others, but each culture has its norms that have worked for it. The norms preserve the herd. They are hard to criticize as immoral. Eugenics is about forcing people to mate in a certain way. It’s a bad idea for reasons relating to power dynamics more than anything else. (The droit du seigneur was essentially an exercise in eugenics.) But social norms that tie sexual selection to visible soundness remain an arguably innate strategy mediated by our most selfish genes.

That’s a serious headwind for “body positivity” to buck.