In the Old Testament, those who were called the “sons of God” were:
Adam (Luke 3:38),
The Angels (Job 1:6, 2:1, 38:7), and
Israel (Isaiah 43:6,7, Psalms 82:6, Matthew 9:2,22).
Adam lived 930 years (Genesis 5:5), and we can see by the geneaologies in Genesis 5 that the time of 120 years before the flood (Genesis 6:3) was approximately 1,000 years after the death of Adam.
And though Adam was verily created in the image of God, he sinned and fell, and afterward begat sons “in his own likeness, after his image” (Genesis 5:3) as a sinner (Romans 5:12); and as a result his children were not called in the scripture “sons of God”.
Seth was given by God as a substitute for the seed of Abel, whom Cain slew, but, nowhere are Seth or his seed called “sons of God”…
The scripture gives us no reason to believe that Seth or his seed possessed any special power to produce or engender “giants” (Genesis 6:4).
Israel came from Abraham, who did not yet exist during the time indicated in Genesis Chapter 6.
Therefore we can see clearly that “the sons of God” in Genesis 6 were NOT men.
So, the only other possibility is that they were angels.
Peter and Jude spoke of them in the New Testament (2 Peter 2:4,5, Jude 1:6,7), saying that they had fornicated after the same manner as Sodom and Gomorrha; and they are not running to and fro through the earth today, as are the devil and his angels (Job 1:7, 2:2, Ephesians 6:12), but are rather “…reserved under chains of darkness unto the judgment of the great day…”
They are in the abyss where the unclean spirits of today fear to go (Luke 8:31).
After his crucifixion, our Lord Jesus Christ preached his victory to them (1 Peter 3:19, Colossians 2:15).
They are the “third part of the stars of heaven” that were cast down to the earth by the tail of the dragon (Revelation 12:4).
Of a truth, it is clear that the sons of God in Genesis 6:4 are angels.
Can angels have physical bodies?
The scripture testifies that they can indeed.
In Genesis 18 & 19, the angels washed their feet (18:4), and ate meat and bread (18:8).
They did the same thing in Sodom (19:2,3).
The men of Sodom wanted to know them sexually (19:5).
They reached out their hands, and pulled Lot into the house with them, and shut the door (19:10).
Jacob wrestled with an angel of the LORD (Genesis 32:24-30).
The writer of Hebrews says that some have entertained angels unawares (Hebrews 13:4)…
Can angels, being spirits, beget children?
Some would say no, citing Matthew 22:30. The disobedient angels before the flood “took them wives of all which they chose”(Genesis 6:2), but we can see in Matthew 22:30 that Jesus was not speaking of ALL the angels, but rather, “the angels which are in heaven”. Those that rebelled before the flood are not in heaven, but are in the abyss.
(2 Peter 2:4, Jude 1:6, 1 Peter 3:19,20, Ephesians 4:9, Colossians 2:15)
Also, God himself put his seed into the womb of a virgin (but not by fornication!), and begat a Son by her.
Clearly the Son of God, Jesus Christ, is both the Son of God (Luke 1:35, Romans 1:4), and also the Son of Man (Romans 1:3, Luke 19:10), the seed of the woman (Genesis 3:15).
The scripture says, “There is a natural body, and there is a spiritual body.”
(1 Corinthians 15:44).
If God, being a Spirit (John 4:24), is able to put his seed into the womb of a virgin, and if he is able to take for himself a bride (Israel–Jeremiah 3:14, Ezekiel 16:8), why do so many say that the “sons of God” (Job 1:6, 2:1, 38:7) cannot take to themselves wives and bear children, because they are spirits?
It is clear from the scripture that they can indeed, although it is forbidden. And of a truth they did just that in the days before the flood, and the result was mutants–giants.
This also happened after the flood
Numbers 13:32,33, 1 Samuel 17:4,
2 Samuel 21:15-22;
and it will happen again in the last days
1 Corinthians 10:11,
Watch and pray.