unnatural argument is to be ignored IMHO, it does't makes sense outside our emotional feeling and has multiple (and contradictory) interpretation so you'll end up arguing on each cases and don't have a rule/framework.
A thought experiment from Jonathan Haidt:
> Julie and Mark are brother and sister. They are traveling together in France on summer vacation from college. One night they are stay- ing alone in a cabin near the beach. They decide that it would be interesting and fun if they tried making love. At the very least it would be a new experience for each of them. Julie was already taking birth control pills, but Mark uses a condom too, just to be safe. They both enjoy making love, but they decide not to do it again. They keep that night as a special secret, which makes them feel even closer to each other. What do you think about that? Was it OK for them to make love?
edit: Confusing story isn't it ? Here's an short but thorough analyse https://www.kevindorst.com/uploads/8/8/1/7/88177244/haidt_20...
0 https://en.wikipedia.org/wiki/Appeal_to_nature and https://en.wikipedia.org/wiki/Naturalistic_fallacy