INCEST

[Minimalist]

Because it is black-letter law in every state in the union. 

In all states, close blood-relatives that fall under the incest statutes include father, mother, grandfather, grandmother, brother, sister, aunt, uncle, niece, nephew, and in some states, first cousins. Many states also apply incest laws to non-blood relations including stepparents, step-siblings, and in-laws.

https://en.wikipedia.org/wiki/Black_letter_law

In common law legal systems, black letter laws are the well-established technical legal rules that are no longer subject to reasonable dispute. Some examples are the "black-letter law" of contracts or the "black-letter law" of trademarks. Black-letter law can be contrasted with legal theory or unsettled legal issues.

I would submit that gay marriage fell under the unsettled legal issues doctrine if only because jesus freaks thought they could vote to deny other people their civil rights.