“You wonder what people were doing before Noah’s Flood?” The Bible indicates that they were messing with the genome of humanity, to prevent a clean blood line to the one who could crush satan’s head. That’s why the Bible specifies that Noah was undefiled in all his generations. We have no idea about his sone’ wives, however.
I suspect that at least one of Noah’s daughters-in-law had contaminated DNA but that was not an issue because God knew it would not go very far.
Of course, Scripture seems to indicate that the sons of God who produced the Nephilim tried again at a later date.