For concreteness, imagine a different story where Julie and Mark decide to play Russian roulette in their cabin (again, just for fun). They both miss the bullet, no harm results, and they never tell anyone etc. etc. So what was wrong with their actions?

I think most people would be able to handle that one very quickly. So the really interesting question is why no-one comes up with such an explanation in the incest case.

Expecting evolved moral instincts to conform exactly to some simple unifying principle is like expecting the orbits of the planets to be in the same proportion as the first 9 prime numbers or something. That which is produced by a complex, messy, random process is unlikely to have some low complexity description.

An interesting analogy. I mean, who would predict something crazy like the square of the orbital period being proportional to the cube of the orbital radius?

Obviously there's no unifying principle in all that messy moral randomness. No hidden laws, just waiting to be discovered...