So do we think there's a good case for coercive eugenics here then? That's got a sort of Schelling-pointy feel to it to me, so that I wonder if prohibiting incest might actually be the lesser of two evils.

If we're comparing two "coercions" (coercive prohibition of incestuous couples from having children) (coercive prohibition of incestuous couples from having sex), the former seems to be the lesser coercion by far, and the more easily justified.

So is it "coercive" part of "coercive eugenics" that really bugs you, or is it that you have an ick-factor against eugenics altogether, voluntary eugenics too?