This ignores the reality of incest having a far higher chance than non-incestual pregancies of producing a birth defect.
But I agree, we don't do eugenics on people with Down's or autism, so that justification here is bullshit.
I can't remember the specifics, but it's something like women over 40 have just as much chance of producing a birth defect as incest. The real problem comes when incest is repeated for multiple generations, like royal families did. But that's extremely unlikely without the "royal blood" concept anymore.