Granted. But would you grant that inbreeding is basically the cause of what we call races, and further that racism depends on this kind of inbreeding?

I wouldn't call it inbreeding, since inbreeding requires "close" relations. I suppose if you wanted to stretch the definition to a macro scale then I would agree, however.

Was there a study you know of comparing rates of birth defects between interracial and intra-racial births? Let's suppose hypothetically that todays most common intra-racial pregnancy is 5 times less likely to result in a birth defect than if first cousins mated. If a study showed that interracial pregnancies were 5 times less likely than intra-racial pregnancies to produce a baby with birth defects would this not justify outlawing them by the same rationale that we ban incest?

I think that this is an incredibly dangerous path, as it would advocate banning all non-ideal paths. We ban incestual breeding because of a heightened risk of harm over the norm. Just because we have found (hypothetically) a new method with a further decrease chance doesn't mean that all other forms should be banned outright.

It would be like finding out that white meat is less harmful than red meat, so we ban red meat. I don't want to live in a world that follows logic that would ban red meat.