"What's really wrong with it?"
"You breed too close, you get faulty offspring. Idiots and dead babies without heads and all that."
"I knew it! I knew it!" crowed the big Vexveltian. "Isn't it wonderful? From the rocky depths of a Stone Age culture through all the brocades and knee-breeches sort of grand opera civilizations all the way out to the computer technocracies, where they graft electrodes into their heads and shunt their thinking into a box -- you ask that question and you get that answer. It's something everybody knows. You don't have to look at the evidence."
"Where do you go for evidence?"
"To dinner, for one place, where you'll eat idiot pig or feeble-minded cow. Any livestock breeder will tell you that, once you have a strain you want to keep and develop, you breed father to daughter and to granddaughter, and then brother to sister. You'll keep that up indefinitely until the desired trait shows up recessive, and you stop it there. But it might never show up recessive. In any case, it's rare indeed when anything goes wrong in the very first generation; but you in the bar, there, you're totally convinced that it will. And are you prepared to say that every [mentally challenged person (edited to get through the filter-TM)] is the product of an incestuous union? You'd better not, or you'll hurt the feelings of some pretty nice people. That's a tragedy that can happen to anybody, and I doubt there's any more chance of it between related parents than there is with anyone else."