New Defender's Study Bible Notes
5:4 sons and daughters. Probably many children were born to Adam during his long life; the ancient quibble about “Cain’s wife” is easily resolved in terms of brother/sister marriages in the first generation. Close marriages are genetically dangerous today because of the accumulation of harmful mutations in the human genetic system over many generations, and incest has been prohibited since Moses’ time (Leviticus 20:11-20). In the first few generations, including those after the Flood, marriages of near relatives were necessary in order for mankind to obey God’s command to “multiply” (Genesis 1:28; 9:1), and accumulated mutations were few.