Genesis 6:1-4 The Nephilim (giants)
1 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them, 2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
3 And the LORD said, My Spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
4 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.” [KJV]
Y The timing was not from the beginning but, came about when men began to multiply on the earth. By this time the human race had become a multitude.
Y They were a source of strife with God.
Y They were given a unique life expectancy.
Y The sons of God who married the daughters of men were not normal people as some try to teach, but were apparently angles who left their domain in heaven and took wives of the daughters of men. The text is very specific to point out that this union was unique and the offspring were very unique.
Normally in Scripture, women are not named except when they play a part in the revelation at hand.
I am quite sure that there were marriages between the descendants of Cain and the descendants of Seth from the beginning. Except for Adah and Zillah the wives of Lamech which were named to tell the story of Lamech, the only woman named of the descendants of Cain was, “Naamah” the sister of Tubal-Cain, whom I believe was the wife of Noah.
The fact that angels were called “sons of God” would tend to confirm the concept that the sons of God in this text were angels, especially when you refer to other unmistakable texts that use the same terminology. See [Job 1:6; 2:1; 38:1-7] Where God Himself calls the angels “sons of God.” See also [2 Peter 2:4] where Peter speaking of fallen angels says: “For if God did not spare the angels who sinned, but cast them down to hell and delivered them into chains of darkness, to be reserved for judgment;” NKJV
I have known many marriages between saints and sinners but, they never produced the unique offspring that is described in this passage, “Mighty men which were of old, men of renown.”
God gave them a special time for their life span 120 years because they “also” are mortal which would be self evident if they were normal people and they would not be worthy of such a special reference.
Then, verse 4 expands on the subject as unique when he says, “in those days and also after” that, “when the Sons of God came into the daughters of men, and they bear children to them.” The question is, “Before and after what?” Could it be a reference to the Great Flood which follows immediately after this revelation ?
That would mean that before the Flood angels took wives of human women and it happened again after the Flood which would explain why so many pagan religions, especially of Greek culture, have so many ‘powerful gods in human form’ - - most of whom were supernatural in power and driven by the same emotions as human beings some good but essentially wicked. They lived and they died which is in itself strange for a being that is considered a “god.”
Because there are none of them around today, it would appear that the fact that they were bound by God to be kept until the great day of judgment (as expressed in the book of Jude) may really be the answer both concerning who they were and why they are not here now. “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 KJV]
Moreover, there is no indication in Scripture that angels cannot reproduce sexually. The only such references that are usually pointed out as indicating that angels are sexless, are where Jesus is describing the resurrected saints as being “like the angels as they never marry nor are given in marriage.” [Matthew 22:30] This is expanded in [Luke 20:34-36] where Luke explains what is meant by the term “they will be like the angels.”
“And Jesus answered and said to them, "The sons of this age marry and are given in marriage. But those who are counted worthy to attain that age, and the resurrection from the dead, neither marry nor are given in marriage; nor can they die anymore, for they are equal to the angels and are sons of God, being sons of the resurrection.” [Luke 20:34-36 NKJV]
Notice that the similarity to angels is the fact that they cannot die because they are “sons of God” being sons of the resurrection. This has no reference to angels as being sexless.
It is entirely possible that angels, who are created by God, may reproduce like men who are created by God also and reproduce by the sexual process. IF that is the case, it would be entirely possible for angels to reproduce with women fathering that unique race called the Nephilim (giants) with such supernatural powers and resulting renown.
That would also explain why God would specifically say that “they also are mortal” and would give them a special life span. Where the life span for man is much shorter, “The days of our lives are seventy years; and if by reason of strength they are eighty years, Yet their boast is only labor and sorrow; For it is soon cut off, and we fly away.” [Psalm 90:10 NKJV]