To quote Robert Bolt, channelling St. Thomas More, the world must construe according to its wits. Courts must construe according to the law. I personally don’t see how they allow same-sex marriage between consenting adults and don’t eventually allow polygamous marriages, or for that matter consanguinous marriages, or (oy) any combination thereof, on the same basis. Yeah, there’s still the eugenics thing about incestuous marriages. Do you really want to be the poor slob of an attorney making that argument in an American courtroom in 2014? No. No, you do not.
Particularly since any eugenics argument against allowing incestuous coupling would also apply against allowing long-term welfare recipients to breed.