Adam and eve only proves incest is ok in the bible, adam and eve didnt exist in reality, incest isnt socially accepted, and its not common because opposites attract, opposites attracting is chemical attraction at its best, without knowing it we look for people genitically different from ourselves, the more different the better, its about reproduction and evolution, if you reproduce with someone from the same genepool you dont produce anything 'improved' if you reproduce with someone drastically different from yourself with different imunities, then you could produce a child with all of the adults immunities combined, you dont get that from incest.(as for gay incest its just not socially accepted)