It is only by the Divine specific act of creation that any created being can be called "a son of God". For that which is "born of the flesh is flesh". God is spirit, and that which is "born of the Spirit is spirit" (John 3:6). Hence Adam is called a "son of God" in Luke 3:38. Those "in Christ" having "the new nature" which is by the direct creation of God (2 Corinthians 5:17. Ehpesians 2:10) can be, and are called "sons of God" (John 1:13. Romans 8:14,15. 1John 3:1).1
This why angels are called "sons of God" in every other place where the expression is used in the Old Testament. Job 1:6; 2:1; 38:7. Psalms 29:1; 89:6. Daniel 3:25. (no article). 2 We have no authority or right to take the expression in Genesis 6:2,4 in any other sense. Moreover, in Genesis 6:2 the Septuagint renders it "angels".
Angels are called "spirits" (Psalm 104:4. Hebrews 1:7,14), for spirits are created by God.
That there was a fall of the angels is certain from Jude 6.
The nature of their fall is clearly stated in the same verse. They left their own (oiketerion). This word occurs only in 2Corinthians 5:2. Jude 6, where it is used of the spiritual (or resurrection) body.
The nature of their sin is stated to be "in like manner" to that of the subsequent sins of Sodom and Gomorrha, Jude 7.
The time of their fall is given as having taken place "in the days of Noah" (1Peter 3:20. 2Peter 2:7), though there may have been a prior fall which caused the end of "the world that then was" (Genesis 1:1,2. 2Peter 3:6).
For this sin they are "reserved unto judgement", 2Peter 2:4, and are "in prison", 1Peter 3:19.
Their progeny, called Nephilim (translated "giants"), were monsters of iniquity; and being superhuman in size and character, had to be destroyed (see Appendix 25).This was the one and only object of the Flood.
Only Noah and his family had preserved their pedigree pure from Adam (Genesis 6:9 see note). All the rest had become "corrupt" (shachath) destroyed [as Adamites]. The only remedy was to destroy it (defacto), as it had become destroyed (de jure). (It is the same word in verse 17 as in verses 11,12.) See futher under Appendix 25 on the Nephilim.
This irruption of fallen angels was Satan's first attempt to prevent the coming of the Seed of the woman foretold in Genesis 3:15. If this could be accomplished, God's Word would have failed, and his own doom would be averted.
As soon as it was made known that the Seed of the woman was to come through ABRAHAM, there must have been another irruption, as recorded in Genesis 6:4, "and also after that" (that is to say, after the days of Noah, more than 500 years after the first irruption). The aim of the enemy was to occupy Canaan in advance of Abraham, and so to contest its occupation by his seed. For, when Abraham entered Canaan, we read (Genesis 12:6) "the Canaanite was then (that is to say, already) in the land."
In the same chapter (Genesis 12:10-20) we see Satan's next attempt to interfere with Abraham's seed, and frustrate the purpose of God that it should be in "Isaac". This attempt was repeated in 20:1-18.
This great conflict may be seen throughout the Bible, and it forms a great and important subject of Biblical study. In each case the human instrument had his own personal interest to serve, while Satan had his own great object in view. Hence God had, in each case, to interfere and avert the evil and the danger, of which His servants and people were wholly ignorant. The following assaults of the great Enemy stand out prominently:-
The destruction of the chosen family by famine, Genesis 50:20.
The destruction of the male line in Israel, Exodus 1:10,15, etc. Compare to Exodus 2:5. Hebrews 11:23.
The destruction of the whole nation in Pharaoh's pursuit, Exodus 14.
After David's line was singled out (2Samuel 7), that was the next selected for assault. Satan's first assault was in the union of Jehoram and Athaliah by Jehoshaphat, notwithstanding 2Chronicles 17:1. Jehoram killed off all his brothers (2Chronicles 21:4).
The Arabians slew all his children, except Ahaziah (2Chronicles 21:17; 22:1).
When Ahaziah died, Athaliah killed "all the seed royal" (2Chronilces 22:10). The babe Joash alone was rescued; and, for six years, the faithfulness of Jehovah's word was at stake (2Chronicles 23:3).
Hezekiah was childless, when a double assault was made by the King of Assyria and the King of Terrors (Isaiah 36:1; 38:1). God's faithfulness was appealed to and relied on (Psalm 136).
In Captivity, Haman was used to attempt the destruction of the whole nation (Esther 3:6,12,13. Compare 6:1).
Joseph's fear was worked on (Matthew 1:18-20). Notwithstanding the fact that he was "a just man", and kept the Law, he did not wish to have Mary stoned to death (Deuteronomy 24:1); hence Joseph determined to divorce her. But God intervened: "Fear not".
Herod sought the young Child's life (Matthew 2).
At the Temptation, "Cast Thyself down" was Satan's temptation.
At Nazareth, again (Luke 4), there was another attempt to cast Him down and destroy Him.
The two storms on the Lake were other attempts.
At length the cross was reached, and the sepulchre closed; the watch set; and the stone sealed. But "God raised Him from the dead." And now, like another Joash, He is seated and expecting (Hebrews 10:12,13), hidden in the house of God on high; and the members of "the one body" are hidden there "in Him" (Colossians 3:1-3), like another Jehoshaba; and going forth to witness of His coming, like another Jehoiada (2Chronicles 23:3).
The irruption of "the fallen angels" ("sons of God") was the first attempt; and was directed against the whole human race.
When Abraham was called, then he and his seed were attacked.
When David was enthroned, then the royal line was assailed.
And when "the Seed of the woman" Himself came, then the storm burst upon Him.

NOTES
1 The word "offspring" in Acts 17:28 is quite different. It is (genos), which means merely kin or kind, our genus as being originated by God.
2 In Hosea 1:10, it is not beni-ha-Elohim, as here, but beni-el-chai.