The short answer is YES, the Nephilim (“fallen ones, giants”) were the offspring of sexual relationships between the sons of God and daughters of men in Genesis 6:1–4. There is much debate as to the identity of the “sons of God.” the Biblical accounts only support that they were fallen angels (demons) who mated with human females or possessed human males who then mated with human females. How this manifested is unknown, these unions resulted in offspring, the Nephilim, who were “heroes of old, men of renown” (Genesis 6:4).
As to a distinct motivation of the demons, one speculation is that the demons were attempting to pollute the human bloodline in order to prevent the coming of the Messiah. God had promised that the Messiah would one day crush the head of the serpent, Satan (Genesis 3:15). The demons in Genesis 6 were possibly attempting to prevent the crushing of the serpent and make it impossible for a sinless “seed of the woman” to be born. Again, this is not a specifically biblical answer, but it is biblically plausible.
According to Hebraic and other legends (the Book of Enoch and other non-biblical writings), Nephilim were a race of giants who did acts of great evil. Their great size and power likely came from the mixture of demonic “DNA” with human genetics. All that the Bible directly says about them is that they were “heroes of old, men of renown” (Genesis 6:4). The Nephilim were not aliens, angels, “Watchers,” or rock monsters; they were literal, physical beings produced from the union of the sons of God and the daughters of men (Genesis 6:1–4).
The Nephilim were one of the primary reasons for the great flood in Noah’s time. “And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually. And it repented the Lord that he had made man on the earth, and it grieved him at his heart. And the Lord said, I will destroy man whom I have created from the face of the earth; both man, and beast, and the creeping thing, and the fowls of the air; for it repenteth me that I have made them.” Genesis 6:5–7.
God proceeded to flood the entire earth, killing everyone and everything other than Noah, his family, and the animals on the ark. All else perished, including the Nephilim (Genesis 6:11–22).
Were there Nephilim after the flood? “There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.” Genesis 6:4
The demons repeated their sin sometime after the flood as well. However, it likely took place to a much lesser extent than it did prior to the flood. When the Israelites spied out the land of Canaan, they reported back to Moses: “And there we saw the giants, the sons of Anak, which come of the giants: and we were in our own sight as grasshoppers, and so we were in their sight.” Numbers 13:33 (the descendants of Anak come from the Nephilim)
This passage does not say the Nephilim were genuinely there, only that the spies thoughtthey saw the Nephilim. It is more likely that the spies witnessed very large people in Canaan and in their fear believed them to be the Nephilim. Or it is possible that after the flood the demons again mated with human females, producing more Nephilim. It is even possible that some traits of the Nephilim were passed on through the heredity of one of Noah’s daughters-in-law. Whatever the case, these “giants” were destroyed by the Israelites during their invasion of Canaan (Joshua 11:21–22) and later in their history (Deuteronomy 3:11; 1 Samuel 17).
What prevents the demons from producing more Nephilim today? It seems that God put an end to demons mating with humans by placing all the demons who committed such an act in isolation. “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.”Jude 6
Not all demons are in “prison” today, so there must have been a group of demons who committed further grievous sin beyond the original fall. Presumably, the demons who mated with human females are the ones who are “bound with everlasting chains.” This would prevent any more demons from attempting such sin. To that end, there are many bones of these giants that have been found in the sediment shelves around the world with large deposits in the Grand Canyon. Much of this evidence in within the Smithsonian Institute archives in the US and other institutions around the world.