In the OT (as opposed to the NT) only angels are called "sons of God" (meaning direct creations of God). And the book of Job confirms this.
The book of Job confirms the stars and sons of God were two different types of beings created on Day 4 and Day 6. The Angels were created on Day 4. The sons of God were created on Day 6. They all glorified God at the beginning.
Yes a third of the stars were cast out of heaven because they rebelled. They were immediately bound in the pit. They did not pass earth and collect giant offspring. Besides, Revelation claims when they come out of the pit in the 5th Trumpet, they are locust like creatures, not reptilian or human. Revelation 9:1-5
1 And the fifth angel sounded, and I saw a star fall from heaven unto the earth: and to him was given the key of the bottomless pit.
2 And he opened the bottomless pit; and there arose a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit.
3 And there came out of the smoke locusts upon the earth: and unto them was given power, as the scorpions of the earth have power.
4 And it was commanded them that they should not hurt the grass of the earth, neither any green thing, neither any tree; but only those men which have not the seal of God in their foreheads.
5 And to them it was given that they should not kill them, but that they should be tormented five months: and their torment was as the torment of a scorpion, when he striketh a man.
Jude 1:6
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.
God only opens up the pit for this time of judgment. Genesis 6 is not about angels at all. It was only about how sin corrupted all of the sons of God. This rebellion on earth brought about the Flood on mankind without any help from Satan or the fallen angels. All Satan had to do was get Eve to fall for his deception. Nature took care of the rest of the fall of all mankind.