The phrase "sons of God" appears 4 times in the Old Testament, twice in the passage in Genesis, and twice in the book of Job. Job's passages are plainly angels, so in the context of the Old Testament, the Genesis references should be understood first as angels unless there is sufficient reason to rule it out.
Angels have the ability to take on human form (see Genesis 19:5-8). In the case of these fallen angels, they apparently took on human form complete with tweaked human DNA coded for extreme size and aggressiveness. Angels in heaven do not marry, but it doesn't say that they are incapable of taking on human form and marrying. God's angels who did not rebel have no use or need for human females, so they don't marry. Human females are made to correspond to the human male who is made in the image of God, and this explains their appeal to the fallen angels who were themselves created as "sons of God". (Male and female are together God's image; but man is the direct image while woman is an indirect image according to 1 Corinthians 11:7)
These giants are called "men" in Genesis and elsewhere, indicating that regardless of their satanic origin, God considered them human, and dealt with them and judged them accordingly.
The groups listed by Moses in Deuteronomy are noted for their size. Og was a remnant of these groups, and his bed was about 13.5 feet long by 6 feet wide. This would make him roughly twice as tall as an ordinary person today. This cannot be explained by giantism today, since giantism is debilitating and does not result in "mighty men". These giants were superior athletes.
The spies refer to the sons of Anak as having great size (albeit exaggerated), and by that they claim that they were "Nephilim". So the spies are interpreting the Nephilim in Genesis as meaning giants in terms of size. If they were twice the height of normal men as Og's bed seems to indicate, then their fear and comparisons are understandable (though inexcusable in light of God's power).
There are 5 tribes of giants mentioned in the Bible after the Flood: Emims, Zamzummims, Horims, Avims, and Rephaims. They lived around the perimeter of Canaan. According to Genesis 6:4, they were produced the same way the Nephilim were produced before the Flood. Emims, Horims, and Rephaims (possibly the Zamzummims also) were defeated in the first wave of Chedorlaomer's invasion in Genesis 14:5-6 before he was defeated by Abraham. The giants were weakened in number by this attack. Later the Emims were eradicated from Ar by the Moabites, the Zamzummims were finished in Ammon by the Ammonites, the Horims were driven out of Mt Seir by Esau, and the Avims were finished off by the Caphtorites. Caphtor was a descendant of Mizraim (Hamitic, from Egypt). According to Josephus, their cities were destroyed in the Ethiopic war and so they migrated to Phillistia where they encountered and defeated the Avims.
The Rephaims included Og and the Anakim, the sons of Anak who was a son of Arba. Arba himself may have been one of the first post-flood hybrids since his father is not cited.
These were defeated by the Isrealites as they invaded the promised land.
Joshua 11:22 states that there were giants left in Gaza, Gath, and Ashdod. This explains the later appearance of Goliath from Gath. Goliath was roughly 9.5 feet tall. A few generations removed from Og's size of approximately 13.5 feet tall, the original hybrids were likely the largest, and their descendants got smaller as they interbred.
Jude verse 6 refers to the punishment of angels; the judgment of some angels is reserved for the end in Revelation 12:7-8.
Matthew 22:30 Jesus said that the angels of God in heaven do not marry. He said nothing about what the devil's angels (see Rev 12:7-8) do when they are on Earth.
Genesis 6:4 states that there were "giants in those days, and also after that", and identifies this as a result of the aforementioned interbreeding. Giants after the Flood include 5 tribes mentioned in the Bible. It is highly unlikely that the interbreeding of believers and non-believers after the Flood would produce giants. If the production of giants requires a daughter of Cain and a son of Seth, then a daughter of Cain would have to have married a son of Noah. If that son was Ham, then all of Ham's sons should be giants. The tribes of giants lived around the Canaanites, but none of them are listed as the offspring of Canaan or Ham or anyone else in Genesis 10. This is best explained by the idea that they were generated by fallen angel/human hybridization, so their male lineage was not through a son of Noah.
