1 • When man began to multiply on the face of the land
and daughters were born to them,
2 • the sons of God saw that the daughters of man were attractive.
And they took as their wives any they chose.
3 • Then the Lord said,
"My Spirit shall not abide in man forever,
for he is flesh:
his days shall be 120 years."
Right from the outset, as soon as the forbidden fruit was eaten, a total ending of mankind's life on earth was in view. Mankind in general was not living with imminent death in mind because death seemed to be limited to those who 'called upon the name of the LORD.' [Genesis 4:26] (No death dates are given in Genesis 4 but they are scrupulously recorded in Genesis 5.) Just as God's patience was running to its conclusion and he was setting a date for the end of everything in 120 years time, the sons of God (angels who did not stay within their own position of authority, but left their proper dwelling [Jude 6]) took the opposite view and exchanged life in heaven for the seemingly better and just as long lasting life on earth.
