I should think that it was a taboo long before the church or modern genetics. It was probably observed that small groups of humans whom did not mate outside of the family were more likely to become unwell, a probably scenario when man formed small hunter gatherer groups. This may have been interpreted as vengeance from god(s) for incest or a number of things. The way of thinking would have been around when the bible was written and inserted. Or at least this is my theory. Now for the stuff I can say with cirtainty: Please be careful of the traits you bring into this debate. Sickle cell in Africa is actually a beneficial if you are a carrier (ie one good allele and one sickle allele) as it gives resistance to malaria. When you have two sickle cell alleles you suffer from the disease and die. Also Cystic fibrosis carriers were thought to have a resistance to the black death, but again two bad copies gives disease. The sort of things that occur through insect are the diseases that have no benefit to carry the gene, and they occur because as mutations ac!@#$%^&*ulate in a population they are not being diluted by the input other populations who have good copies of the same gene. It is well worth noteing that one round of insest probably will not cause horribale mutations to surface, it usually takes multiple generations for the effects to be seen.