The sons of God (that is, the descendants of Seth) began to marry the daughters of men (including the descendants of Cain), but was that what God was upset by the most? Polygamy! Maybe that was it? This much is clear: there was a population explosion, a rise of hedonism, a breakdown of familial morality and eventually an absolute depravity of society... worldwide.
Even though the people were fine specimens physically and mentally, they were spiritually and morally decadent. Great! Every! Only! Continually! All! In verses 5 & 12, these words describe the debauched wickedness that permeated the hearts and minds of the human race before the flood. Our ancestors were so horribly offensive to God that He even wished He had never made us! He determined to destroy our race!
Yet despite mankind's universal degeneracy, there was (again) one exception. Glorious words, "But Noah found grace in the eyes of the LORD..." and "Noah was a just man and perfect in his generations, and Noah walked with God" (Genesis 6:8-9). Grace! Noah was chosen by God to "reset" the race.
God came to Bro. Noah and told him 3 basic things:
1. The earth is corrupted with violence,
2. The earth will be destroyed with a flood of water, and
3. Make a lifeboat so that My wrath will not be the end of life on earth.
The Bible is full of covenants; testaments; promises... one of those was made with Noah.
Q: What did Noah have to do as a result of this covenant
A: Obey! (see verse 22)
What about us? The world may not be at an antediluvian-level-of-bad yet, but things are rapidly descending in that direction. In Matthew 24:37 & Luke 17:26 it is recorded that Jesus said, "As it was in the days of Noe, so shall it be also in the days of the Son of man."
Who knows how much time remains? What's important though is this: God has taken the initiative today too! He has offered grace to us too? Yes! Jesus is our ark; our lifeboat. But, the next annihilating rain will be one of fire, not of water.
Are we responding in obedience like Noah did? God's covenant with us should motivate us to obey God's commands (II John 1:5-6).