It is true, crossing close relatives will increase the likelihood of undesirable recessive genes being expressed. But those gene have to be there in the first place before they can surface.
As an example, in a family I have known all my life, the Mom has a wandering right eye.
But none of her kids exhibited this trait. Then they let the Mom's 13 year old nephew come up to the ranch for the summer. The nephew and a 13 year old daughter did what kids naturally do when they get the chance to be alone behind the haystack.
And a lovely baby boy resulted, and yes he had Gramma's wandering right eye. Even though neither of the cousins expressed the trait, they did carry the recessive gene inherited from their respective mothers, and the baby was the 1 in 4 chance of inheriting the recessive from both parents.
I am kind of with Heinlein on this one. There is nothing really morally wrong with such a union. But it is immoral to play genetic roulette with any children born of the union.
And today, there is no need. Any decent geneticist can tell couples if it is safe to procreate. And a host of options are available for couples to have children if it is unsafe for them to mate.
