Gen 6: 4 In those days, and also afterward, there were giants on the earth, when God's sons had come to the daughters of men and bore them children for them; these are the giants of old, men of renown.
Read the Bible literally!
Judas 1:6-7 6 En de engelen die hun oorspronkelijke staat niet hebben bewaard, maar hun eigen woonplaats verlaten hebben, heeft Hij voor het oordeel van de grote dag met eeuwige boeien in de duisternis in verzekerde bewaring gesteld.
7 Evenzo is het met Sodom en Gomorra, en de steden eromheen, die op dezelfde wijze als zij hoererij bedreven hebben en ander vlees achterna zijn gegaan. Zij liggen daar als een waarschuwend voorbeeld, doordat zij de straf van het eeuwige vuur ondergaan.
Jude 1:6-7 6 And the angels who have not kept their original state, but have left their own dwelling place, he has placed in sure custody in darkness with everlasting fetters before the judgment of the great day.
7 Likewise it is with Sodom and Gomorrah, and the cities around them, which committed fornication in the same way as they did and followed other flesh. They lie there as a warning example in that they are suffering the punishment of eternal fire.
The fallen angels, in their sin, chose to leave their immortal heavenly body and become "flesh," laden with sin and destined for God's judgment.
Jude compares the fallen angels to the men of Sodom and Gomorrah: read Genesis 18:20: Further, the LORD said, The cry of Sodom and Gomorrah is great, and their sin very heavy.
1 The two angels arrived in Sodom in the evening, while Lot was sitting in the gate of Sodom. When Lot saw them, he rose to meet them, and bowed his face to the earth.
2 He said, Behold, my lords, yet depart from your way, and come to the house of your servant, and spend the night there, and wash your feet; tomorrow morning you may rise and continue your journey. But they said: No, we will spend the night in the square.
3 He urged them strongly, however, so that they turned from their road toward him, and entered his house. He set up a meal for them. He baked unleavened bread and they ate.
4 Even before they lay down to sleep, the men of that city, the men of Sodom, from young to old, surrounded the house; all the people, none excepted.
5 They called to Lot and said to him, Where are those men who came to you last night? Bring them out, to us, so that we may have fellowship with them.
6 Then Lot went out to them at the doorway and closed the door behind him,
7 and he said, My brothers, do no evil!
8 Behold, I have two daughters, who have not had intercourse with any man; yet let me bring them to you, and do with them what is good in your eyes. Only, these men you must not do anything to, for for this reason they have come under the protection of my roof.
9 Then they said: Move aside! Also they said, This one has come to stay here as a stranger, and now he surely wants to be judge over us! Now we will do more harm to you than to them. They pressed very hard on the man, on Lot, and approached to break down the door.
10 But those men put out their hand, pulled Lot toward them into the house, and closed the door.
Sodom was morally ruined. Open sexuality had become the norm! See Rom 1......
So the issue here is sexual sin. And that was exactly the same thing the fallen angels did to human women. Hence the comparison of Jude (and also 2 Peter 2:4-7).
Sex with human women was what God had forbidden, as was sex with animals or homosexuality.
All this eventually led to God's judgment on the earth: the flood.
Biblical evidence of angel-human relationships
How could an angel acquire offspring from a woman? Although the Bible does not give technical details, we can see in other places that angelic reproduction was possible.
Heaven was defiled because the fallen angels were still there....
Job 4: 17 Would a mortal be righteous before God?
Would a man be pure before his Maker?
18 Behold, even His servants He does not trust,
And His angels He charges with error.
The word error here is Nêbalah and means folly, perversion or dishonor.
In Genesis, Deuteronomy, Judges and 2 Samuel this word is always associated with rape, fornication and incest. And now read Job 4: 18 again!
As mentioned, Satan's intention was to corrupt the Messianic bloodline with defiled seed.
But how was that done physically?
If we read in Genesis 18:1-8 we see that the angels took on human forms and also behaved like humans: drinking water (milk), washing feet, resting and finally eating (meat and bread). So angels have biological sytems, just like humans.
(Incidentally, humans can also eat food from angels: Psalm 78:24-25... Manna!)
24 He rained manna on them to eat and gave them heavenly wheat.
25 Everyone ate the bread of the mighty; mighty = angels.
So angels looked like people and functioned as such: Daniel 8:15:
It happened, when I saw the vision - I, Daniel - that I tried to understand it. And behold, there stood before me one with the appearance as of a man.
Also in Luke 24:17 you see a human appearance of angels, as does Mary in John 11:12
Hebr 13:1-2 therefore says that we can "host angels without knowing it"!
Angels have seed
So now that angels can have physical bodies and move around the human environment, does that mean they can reproduce?
1 Cor 15 shows that they also have "seed":
37 And whatever you sow, of it you do not sow the body that will become, but a bare grain of wheat, according to how it comes about, of wheat or of one of the other kinds of grain.
38 God, however, gives to it a body as He has willed, and to each of the seeds its own body.
39 All flesh is not the same flesh, for the flesh of men is different, and the flesh of animals is different, and that of fish is different, and that of birds is different.
40 And there are heavenly bodies and there are earthly bodies, but the glory of the heavenly is different, and that of the earthly is different.
In short, every living thing that has a body also has "seed."
But doesn't the Bible say that angels cannot marry?
In past centuries, there was little talk of angels being able to marry women. It was too bizarre.
We read Matthew 22:23-31:
23 On that day Sadducees came to Him, saying that there is no resurrection, and they asked Him:
24 Master, Moses said, If anyone dies who has no children, his brother must marry his wife and beget offspring for his brother.
25 Now there were with us seven brothers; and the first married and died; and because he had no offspring, he left his wife to his brother.
26 So also the second and the third, up to the seventh.
27 Finally, after all of them, the wife also died.
28 In the resurrection then, of whom of those seven will she be the wife? For they all had her as their wife.
29 But Jesus answered and said to them, You err, because you do not know the Scriptures nor the power of God.
30 For in the resurrection they do not take or give in marriage, but are like angels of God in heaven.
31 And as for the resurrection of the dead, you have not read what was spoken to you by God when he said:
32 I am the God of Abraham and the God of Isaac and the God of Jacob? God is not a God of the dead, but of the living.
So angels do not marry in heaven!
But there do appear to be female angels in the Bible.
Zechariah 5:9...I lifted up my eyes and saw, and behold, two women came forth with the wind under their wings. They had wings like the wings of a stork, and they lifted up the ephah between the earth and heaven.
Disobedient angels rebelled against God and did everything that God forbade: thus also marrying....