Gen 6
1 When men began to multiply on the face of the ground, and daughters were born to them,
2 the sons of God saw that the daughters of men were fair; and they took to wife such of them as they chose.
3 Then the LORD said, "My spirit shall not abide in man for ever, for he is flesh, but his days shall be a hundred and twenty years."
4 The Nephilim [giants KJV] were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. These were the mighty men that were of old, the men of renown.
The phrase "sons of God" appear three more times in the OT (Job 1:6, Job 2:1, Job 38:7). In the OT, the "sons of God" always refer to angels, direct creations of God (let the Bible interpret the Bible). In Noah's case, it is fallen angels who disobeyed God and took on a physical form to breed with women. Is that possible? Did that happen? Yes.
Jude 1:6
And the angels that did not keep their own position but left their proper dwelling have been kept by him in eternal chains in the nether gloom until the judgment of the great day;
1 Pet 3:19,20
By which also he went and preached unto the spirits in prison; Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
Heb 13:2
Be not forgetful to entertain strangers: for thereby some have entertained angels unawares.
2 Pet 2:4
For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
After the flood
"The Nephilim were on the earth in those days, and also afterward".
So, where in the OT (after the flood) are these Nephalim and Rephaim giants mentioned again?
Answer - Goliath (1 Sa 17:4) and all the "-ite" and "-im" tribes. (Deu 2:10,21, Deu 3:11, Deu 9:2, Num 13:33)
Examples of giants (1 cubit being about 0.5 m):
1 Sa 17:4
And there went out a champion out of the camp of the Philistines, named Goliath, of Gath, whose height was six cubits [3 m] and a span.
Deu 3:11
(For only Og the king of Bashan was left of the remnant of the Rephaim [giants KJV]; behold, his bedstead was a bedstead of iron; is it not in Rabbah of the Ammonites? Nine cubits [4.5 m] was its length, and four cubits [2 m] its breadth, according to the common cubit.)
Num 13:32,33
So they brought to the people of Israel an evil report of the land which they had spied out, saying, "The land, through which we have gone, to spy it out, is a land that devours its inhabitants; and all the people that we saw in it are men of great stature. And there we saw the Nephilim (the sons of Anak, who come from the Nephilim); and we seemed to ourselves like grasshoppers, and so we seemed to them."
Goliath was about 3 m tall. King Og's bed was about 4.5 m long and 2 m wide. This should make him at least 3 m tall. No wonder the the Jews felt like "grasshoppers" compared to them.
Gen 6:9
...Noah was a righteous man, blameless in his generation; Noah walked with God.
"blameless" here is "tamiyn" (Greek) - physically perfect/without blemish/100 per cent human. It does not mean morally perfect.
Putting it all together
1. Fallen angels had contaminated the human race by mating with human females who produced the giant Nephalim and Rephaim race. Why? Because Satan knew that the Saviour would come from a woman, and that He had to be fully man, not half man, half angelic. We see something similar to this happening again today - human cloning.
2. God did us a favour by wiping out a contaminated human race. He wasn't being cruel when he sent the flood. And he saved Noah partly because he found only Noah "without blemish" ("tamiyn") - fully human.
3. Later, after the flood, these giants surfaced again. Looks like some fallen angels did it again. That's why we read about God telling David to wipe out villages, not sparing women and children. This time, instead of a flood, God raised up Israelis like David to wipe them out. Again, God was not being cruel to human, but doing them a favour.
4. What happened to those "sons of God" who did those terrible things?
God has bounded them in chains and locked them up in a special place in hell called "tartarus".
What happened to the fallen sons of God?
Jude 1:6
And the angels that did not keep their own position but left their proper dwelling have been kept by him in eternal chains in the nether gloom [under darkness KJV] until the judgment of the great day;
1 Pet 3:19,20
By which also he went and preached unto the spirits in prison; Which sometime were disobedient, when once the longsuffering of God waited in the days of Noah, while the ark was a preparing, wherein few, that is, eight souls were saved by water.
2 Pet 2:4
For if God spared not the angels that sinned, but cast them down to hell [Greek "tartaros" - the deepest abyss of hell), and delivered them into chains of darkness, to be reserved unto judgment;
What did Jesus do in 1 Pet 3:19? He was not preaching salvation to men who had died without Christ - as if He was giving the dead a second chance. No, he was essentially proclaiming to the fallen angels who were bound there in darkness: "You tried to stop the Son of Man from coming by contaminating the human race. But you have failed and I have triumphed!"
These sons of God are still there, awaiting their day of judgement.