People began to multiply and the sons (small s) of God (big G) took wives from the daughters of man. IMHO these are fallen angels which were able to take the form of man, here are some reasons I believe this;
(1.) Job 1:6 Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them.
Here we see the same terminology used yet these sons of God are in heaven and while the sons in this passage could be either fallen angels or faithful angels the sons in Gen. 6 would almost certainly be fallen angels.
Job 38:7 When the morning stars sang together, and all the sons of God shouted for joy?
Same words only this time it is God speaking to Job.
In verse 4 we see that the offspring of these unions were giants and that they became mighty men and men of renown. Mighty men in strongs concordance is;
Intensive from the same as H1397; powerful; by implication warrior, tyrant: - champion, chief, X excel, giant, man, mighty (man, one), strong (man), valiant man
And men of renown is;
A primitive word (perhaps rather from H7760 through the idea of definite and conspicuous position; compare H8064); an appellation, as a mark or memorial of individuality; by implication honor, authority, character: - + base, [in-] fame [-ous], name (-d), renown, report.
Personally it seems very possible that the sons of Noah and perhaps Noah himself told stories about these men and in fact it seems there were some of these after the flood until the time of David and I don't think it much of a stretch to believe that the tales of the gods of Greek and Roman mythology came from these people. When Alexander the great conquered the Medes and Persians he adopted their religious practices to better assimilate them into his kingdom and no doubt these deities were known even before that because from Babylonian times this area of the world was very influential up to Greece and east to India. There will be more about this later but it is my belief that the mystery Babylon of revelations was born in these times and has been passed around the world to various nations through the centuries.
(2.) I believe that these angels were punished and that is why we don't see this happening anymore, read this in Jude
Jud 1:6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
While we don't know when this happened (the punishment) we will see that there were giants up until the time of David as we go along. Who ever these sons of God were it is clear that as people increased so did wickedness and God was not pleased. Remember God had said His creation was good but since the fall He was now sorry He had created it, BUT Noah found GRACE in the eyes of the Lord.
Verse 9 says that Noah was perfect in his generations and that he walked with God. While I don't remember the teacher I was listening to at the time, whoever it was had a theory that sounds right to me. He believes that the reason that the fallen angels took wives of humans was to corrupt the human race and therefore prevent the Messiah from being born because of this corruption of the original seed. He believes that Noah's family line was perfect in that nobody in it had been corrupted by the fallen angels and that and the fact that Noah walked with God was why Noah was chosen to build the ark and why the rest of humanity had to be eliminated, to prevent the spread of the corrupted seed and preserve the seed of God's original creation.
And God said unto Noah, The end of all flesh is come before me; for the earth is filled with violence through them; and, behold, I will destroy them with the earth.
The earth is filled with violence THROUGH them. This speaks to me that though man has fallen he still had dominion in the earth and the earth had little choice but to follow mans lead. Therefore if man was violent, the earth (animals, etc..) would be violent because it was under man, and stayed that way until Jesus who has yet to fulfill this;
Rev 11:15 And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign forever and ever.
Rev 11:16 And the four and twenty elders, which sat before God on their seats, fell upon their faces, and worshiped God,
Rev 11:17 Saying, We give thee thanks, O Lord God Almighty, which art, and wast, and art to come; because thou hast taken to thee thy great power, and hast reigned.
Therefore we are in a transition period now that more resembles what was than what will be. Personally I believe another reason Noah was chosen was that he was alive for 84 years before Adam's grandson died and would have certainly heard stories about creation from Adam through him or even possibly received a written record of it through his ancestors.
God tells Noah of His plans and instructs Noah to build the ark, allowing Noah to participate in his own and his family's salvation, which is instructive to us as well because though none can save themselves everyone to be saved must participate by believing and telling others they believe. There are those that teach universal salvation, that everyone will be saved, and there are those that teach predestination, true in that those that are saved are foreknown to God, but not true that they are saved without their knowledge or cooperation. I say it like this, it is our job to tell the truth, it is their job to believe the truth, and it is God's job to save.
Anyway God gives Noah His instructions and promises to establish His covenant (vs. 18) with him. Unless you count Adam and Eve, which I don't, this is the first covenant mentioned in the bible. Vs. 22 Thus did Noah; according to all that God commanded him, so did he.