The two humanities continued: “Eve bore a son and named him Seth,

'For God has appointed another seed for me instead of Abel.'”  We see here the godly line that was Abel,  who was murdered by Cain,   being continued through Seth.  Indeed after Seths's son Enosh was born  “men began to call on the name of the LORD” (Genesis 4:25, 26). It didn't last long, since  “it came to pass, when men began to multiply on the face of the earth.... that the sons of God saw the daughters of men, that they were beautiful; and they took wives for themselves of all whom they chose” (Genesis 6:1, 2). The phrase 'the son of' in Scripture essentially means any descended.  Thus Christ is David's son (Matthew 1:1).  According to the genealogy in Luke 3:38, Seth is the son of Adam,  the son of God. So then Seth is a son of God, and the sons of God in 6:1 above,  are his godly descendants  (see also John 1:12). The sons of men then,  are the other line.  

Concerning the mixed marriages in 6:2 Ellicott  notes “when they married for mere lust of beauty, universal corruption soon ensued.” In fact three verses later we read “Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually (6:5).  The godly line always seems to be in danger of switching and  becoming corrupt.   Paul warns “Do not be misled: "Bad company corrupts good character" (1 Corinthians 15:53). Also  “when Solomon was old, his wives turned his heart after other gods” (1 Kings 11:4);   It can go the other way of course, but that is the exception rather than the rule. Perhaps this is why Paul command that we not be unequally yoked with unbelievers,  as in becoming one flesh in  marriage with them (2 Corinthians 6:14).

In 6:3 we read “And the LORD said, 'My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years.'” The earlier extremely long lifetimes (i.e. Genesis 5:27) are some what puzzling, but see “Questions from Genesis” by Hugh Ross, Chapter 15.  Ellicott again “As the descendants of Seth are now the fallen race, it is their covenant Jehovah who determines to reduce the extreme duration of human life to that which, under the most favourable sanitary influences, might still be its normal length, ” (see also Psalm 90:10). Matthew Henry writes “The Spirit of God strove with men, by sending Enoch, Noah, and perhaps others, to preach to them; by waiting to be gracious, notwithstanding their rebellion.” As we read in Peter,  God is not desirous that any should perish, but all come to repentance and hence eternal life (2 Peter 3:9).

Father, You are slow to anger abounding steadfast love, but there is a limit to Your patience. You wait for us so that You can be gracious to us, but You are a God of Justice (Isaiah 30:18).  In the end then, when it is clear You have been rejected,  You judge righteously. The   fact is however,  that all  have sinned and fallen short of Your Glory (Romans 3:23). In the end You do not send anyone to hell,  and the gift of freewill will be a curse to those who reject Your free gift of eternal life (Romans 6:23)!  Sometimes Lord this overwhelms me. Give me grace to deal with it Lord, and help me live a life that attracts other to You in Jesus Name Amen