(September 1, 2015 at 6:43 pm)Minimalist Wrote: Because it is black-letter law in every state in the union.
Ok, but don't you see how that is a fallacious line of reasoning? Effectively an appeal to tradition.
Just because a law is considered to be well established doesn't mean it should never be challenged, or never considered unjust.
Blasphemy laws were on the books for centuries before being revoked in Europe.
So, without resorting to tradition and current laws (e.g. it should be illegal because it's currently illegal), why should siblings be prohibited from marrying?