I'll try to keep this short because it's late and I'm tired.
Noah and his family were the only faithful humans living at that particular time. The whole earth was corrupt. God had regrets about making mankind. In the final analysis, God could have wiped out the human race with Adam and Eve, but he did not. The Word, Christ, pointed out in Proverbs, that he found love and joy in the creation of man, even though we are imperfect. God would not let Satan hinder his will for the human race but neither could he go through thousands of human pairs to make it happen.
Adam’s line of flesh is under a covenant with God. When the fallen angels materialized, they took on flesh, but it did not come under the covenant because it was not of the Adamic line of descent. This was an automatic death sentence for the children of the angelic fathers and mothers. They had no covenant with God. It was like being born out of wedlock and not knowing who your father was. The Nephilim were considered outcasts in God's eyes, something he felt should never have happened.
Genesis 6:3: "Then the LORD said, "My Spirit will not contend with man forever, for he is mortal; his days will be a hundred and twenty years."
The calling on the name of God in the days of Enoch was not considered good because some of those fallen angels still considered themselves part of the Holy Family despite the fact they had left their duties and forsook their dwelling place (Jude 6). Those fallen angels developed lustful intentions for both men and women companions and took them as partners (Genesis 4:26) Perhaps they thought if humans called upon God's name on their behalf, God might accept their comingling and breeding among the human race. God did not want them as humans. If he had, he would have granted them the right to become humans but they lowered themselves in stature to become less than what they were. God forced the demons to return by flooding the Earth. The flood served as a dual purpose. It was also a judgment for those living on the planet.
