I don't believe we need to get to these questions, because, like the law, morality does not rely on an "every case" principle.

In many states, it is illegal to own "burglary tools." Now, every one of those tools COULD be owned and framed as art, or used in some harmless hobby, or to test one's own security. NOT EVERY owning is "wrong." But every owning is illegal, because the innocent provide cover for the guilty.

Consider mask mandates. I have been fully vaccinated against COVID. Assume that the vaccine is 100% effective against disease and transmission. I can go wherever I want without a mask and neither become sick or make anyone else sick. Yet, it STILL makes sense to require that I wear a mask indoors so that people who are not vaccinated cannot go maskless by pretending to be fully vaccinated. Under that logic, it is "wrong" for me to go maskless indoors, because, if I do, I provide cover for the unvaccinated, even though my action is in itself harmless.

Because incest CAN lead to the Hapsburg chin, or to child abuse, society is served by abhorrence of it as a practice. We cannot have "legitimate" liaisons between siblings or between parents and children, because the resulting tolerant and unjudging atmosphere will make harmful incestual relations too likely to occur. So we just call it "wrong" and move on, with the "good" would-be practitioners just having to take one for the team.