And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,What descendants of Cain? They were all killed in the flood.
2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
3 And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
at this point and tim the flood had not took place ............ i think you better go read more closely Gen 6 is pre flood