Adam and Eve when created were physically perfect. When sin was introduced their genes were soiled to the point that they would "surely die"... from this point every subsequent generation had an increase in risk of genetic mutation in children conceived among siblings. When it reached the point that human genetics were so screwed up that mutation chances were high, God told the people to not mate with their close relatives. (would've added the verses, but i'm both lazy and sure nobody would have checked up on them anyway)