Livre d'Or

I had come up with a couple I thought were good, and they are essentially covered and well more in this comment - I was going for ones on which I hold fairly strong positions that I know are not Western social norms, fwiw.

One could query whether you get a better argument from "Is consensual incest between adults wrong ?" than from the assumptions inherent in having a "why" at the beginning, but that is probably not a discussion I'd want to get into with twelve-year-olds.