It takes *generations* of inbreeding to cause birth defects.

Incest usually doesn't cause birth defects, but it does slightly increase the rate of birth defects. IIRC, something like from 3% to 5%. Because these birth defects are more likely after generations of inbreeding, I am just as glad that incest is deprecated, though I do believe it should be legal as long as it is consensual.

And even if it did, are you going to start prohibiting genetically blind and deaf people from reproducing because they're more likely to have birth defects?

When I consider this from the point of view of the right of the deaf and blind to procreate, I can agree with reproductive rights. When I consider this from the point of view of children who will be permanently and seriously disabled because of the choice of their parents, I am not so sure about this.

Fortunately, with modern medicine and test tube babies we may be able to come up with a solution that allows genetically blind and deaf parents to have seeing and hearing children. When the technology reaches the point where it is cheap and reliable, I would favor a law requiring parents with a genetic defect that disables one of the five senses (seeing, hearing, tasting, smelling, feeling) or which drastically decreases life expectancy (to under 35 years) or which drastically disables them physically (being unable to move without assistance, missing arms or legs or hands or feet) to only reproduce by such artificial means, in order to screen out these serious genetic defects. As far as I'm concerned, the right of children to be able to live a reasonably independent life outweighs the right of parents to duplicate themselves.

That said, I would oppose any laws requiring screening for genetic conditions that are not seriously disabling, and would oppose any laws requiring screening for mental conditions. In both cases because there is generally value in diversity (though I don't believe in diversity for diversity's sake, thus the exceptions), and in the latter case also because the psychiatric profession is still in its infancy, and may not be qualified to even decide what a mental disability is for at least another century.