The phrase ‘sons of God’ refers to faithful people in the following verses (among others):
“You are the sons of the Lord your God.” (Deuteronomy 14:1)
In Christ Jesus you are all sons of God, through faith. (Galatians 3:26)
We are debtors, not to the flesh, to live according to the flesh. For if you live according to the flesh you will die, but if by the Spirit you put to death the deeds of the body, you will live. For all who are led by the Spirit of God are sons of God. (Romans 8:12-14)
The similar phrase ‘children of God’ also refers to faithful people in the following verses (among others):
To all who did receive him, who believed in his name, he gave the right to become children of God (John 1:12)
Do all things without grumbling or questioning, that you may be blameless and innocent, children of God (Philippians 2:15)
See what kind of love the Father has given to us, that we should be called children of God; and so we are…Beloved, we are God’s children (1 John 3:1-2)
So it is consistent to interpret sons of God as faithful people. Then the daughters of man are presumably by contrast unfaithful people. The phrase ‘children of man’ does indeed carry that flavour in several passages:
The faithful have vanished from among the children of man…On every side the wicked prowl, as vileness is exalted among the children of man. (Psalm 12:1,8)
I lie down amid fiery beasts—the children of man, whose teeth are spears and arrows, whose tongues are sharp swords. (Psalm 57:4)
The heart of the children of man is fully set to do evil. (Ecclesiastes 8:11)
The hearts of the children of man are full of evil, and madness is in their hearts while they live, and after that they go to the dead. (Ecclesiastes 9:3)
“All sins will be forgiven the children of man, and whatever blasphemies they utter” (Mark 3:28)
Genesis 6:2 mentions only faithful sons and unfaithful daughters; this is not because there were no faithful women or unfaithful men, but because it is about marriage and marriage was customarily initiated by the man.
A rather odd but not uncommon suggestion is that the sons of God were angels intermarrying with humans. However, Jesus Christ vetoes this interpretation of Genesis 6:2 with his statement that angels do not marry (Matthew 22:30).
There were mighty men in those days, but that doesn’t mean they were offspring of angels. The Hebrew terms of Genesis 6:4 are used for many others at various times long after the flood: Nephilim (giants) in Numbers 13:33, men of renown in Numbers 16:2; 1 Chronicles 5:24; 12:30 and mighty men in Genesis 10:8-9; Joshua 6:2; 1 Chronicles 12:1,4,8,21,25,28,30, etc. None of these people were born of angels either.
God’s response in Genesis 6:3 is to warn of the coming destruction of man; it is man that is at fault, not angels (who cannot sin anyway (Psalm 103:20; Hebrews 1:14); if they could sin, they would also die (Romans 6:23), but angels cannot die (Luke 20:35-36)). So the context of Genesis 6 confirms these sons of God are people — people who had been faithful, but who compromised themselves by marrying ‘any they chose’ (Genesis 6:2).
This failure of God’s people is the lesson Jesus Christ draws from the incident as particularly relevant to people in the day of his return: there is more to think about in life than attractive women and other sensory pleasures.
“Just as it was in the days of Noah, so will it be in the days of the Son of Man. They were eating and drinking and marrying and being given in marriage, until the day when Noah entered the ark, and the flood came and destroyed them all.” (Luke 17:26-27)