User blog comment:Dryuuu/Morgana knowing how Arthur was born/@comment-4814726-20120204161331/@comment-6905051-20130416060229

Cousin Cousin relationship is not frowned upon and in royalties and certain cultures it is still practised. Although depending upon cultures, you cannot marry your paternal first cousin or your maternal first cousin.

That is because when you are cousin, atleast one parent is different, bringing a different gene pool. However when its incest, you are having the possibility of creating a new life from same gene pool, which often results in genetic disorders.

I think in early days of civilization, when things like gene etc were not known,  such anomaly must have been noticed. Early  Egyptians had a trend of marrying sibilings. However science was quite advanced then and people must have observed  the abnormality or disorder that results in inter marriage. Hence relationships between sibilings  was forbidden. Since common people were not so scientifically inclined, societal rules, myths were introduced. That is what I think.

many aboriginal races around the world still practise sibiling relationship and they suffer consequently from many disabilities,  diseases and are gradually wiped out.

The wider range of gene pool is there, the offspring has a better chance of survival over all. Not to mention good looking :)