in those days, death had just come in to the world, so people were healthier (some lived to 900 years).
also the whole of the world's genetic material was encoded in adam and eve's children, so there was an abundance of genes and it was not the same situation as when cousins get married today and there is an increased risk of genetic problems.
Comments
If it speaks of various accidents (personal differences) of human nature (for instance, eye color), there is no reason not to allow it.