There was a study about instinctive mores, featuring the story of Julie and Mark (an adult sister and brother who go camping, have sex, decide not to do it again, but are not harmed by the encounter), and not only did subjects assert such a coupling was morally wrong, but would seek out reasons to justify their belief, even if it didn’t fit the specific circumstances.

I just learned this is a criminal offence in the UK as well. Ridiculous. State absolutely should not get involved into what two consenting, non-vulnerable adults are doing. I could understand if conceiving a child in such relationships was a criminal offence but not the sex itself.