I think the reasons humans have a taboo is not mostly because of possible birth defects.
Some of the birth defect hype is a myth. For example, the fabled Appalachian type “inbreds” that were the basis for movies like Deliverance... But studies have shown that any unusual rates of birth defects in remote poverty stricken areas are far more likely attributable to fetal alcohol syndrome than inbreeding.
Another interesting thing that calls the whole inbreeding hype into question is the fact that as recently as 200 years ago, villages and settlements were typically so small and isolated that the average person met less than fifty people in their entire lives.
Think about that. If there were only fifty people in a typical village, then wouldn’t they all have to be related?
The fact is that only recently has increased population density and job relocation spread us out to the point where it is statistically unlikely that your neighbors or classmates will be cousins.
The further back you go, the more likely it is that your ancestors married their cousins.
So, we all come from a long line of inbreeders.
And look at us. We turned out juld figsplec, tcer splec yhloughl?