Circumstances were different in earlier times. Adam and Eve were created perfect, and the instruction for them ‘to multiply and fill the earth’ would necessarily require that their offspring marry one another and reproduce. (Gen. 1:28) But as perfect humans, their children would have been perfect as were their parents. Adam lived several hundred years remember...

Even though Adam and Eve sinned and became imperfect, Cain and his brothers and sisters were still so near to physical perfection that the children they produced did not suffer the same adverse effects as do children born of such unions today.They lived long too.Even some 2,000 years afterward, God’s faithful servant Abraham married his half-sister Sarah, and God did not disapprove. (Gen. 20:12) It was yet another 450 years or so before God saw fit to provide his nation of Israel a body of laws that forbade incest on penalty of death. (Lev. 18:8-17) By that time imperfection had apparently developed to such an extent that no longer was it safe for close relatives to marry. It is interesting to note that in the bible the average live span of humans started high and steadily declined to a point where man lives 70, 80, or more years with special mightiness.