Gen 6:2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
The sons of God are fallen angels, which rebelled and sinned against God. These fallen angel are the spirits of darkness, and satan is their leader. Fallen angels saw the daughters of men that they were fair, and they took them wives.
Job 2:1 Again there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them to present himself before the LORD.
Job 2 is telling us clearly that the sons of God are angels, in which part fell into sin by rebelling against God, and satan is the leader of fallen angels.
4 ¶ There were giants (nephilim) 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.
5 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.
In the original text is the word nephilim, which comes from the word naphal, which means to fall. The giants were descendants of fallen angels and the daughters of men. Before the Flood, the sons of God (fallen angels) had the sexual relationship with the daughters of men, who bore children to them. Text says that also after that were giants in the earth, that means that fallen angels had the sexual relationship with the daughters of men also after the Flood. From verse 4 we can see that also after the Flood the sons of God (fallen angels) had been sexual intercourse with the daughters of men, who bore children to them and like this, there were giants in the earth after the Flood.
Num 13:33 And there we saw the Nephilim, the sons of Anak, who come of the Nephilim; and we were in our own sight as grasshoppers, and so we were in their sight.' (JPS 1917)
Num 13:33 proves that also after the Flood fallen angels had been sexual intercourse with the daughters of men, and they bore children to them. Num 13:33 says that Israelites saw the giants in the land of Canaan, the sons of Anak, who descended from the giants (fallen angels).
Jewish Publication Society Bible (JPS 1917) is the Bible translated from Hebrew into English by Jewish rabbis. JPS 1917 Num 13:33 translates according to the original text: we saw the giants (nephilim), the sons of Anak, who come of (descendants) the giants (nephilim).
The original text of the Bible proves clearly that fallen angels had been sexual intercourse with the daughters of men after the Flood. Therefore, Gen 6:4 said after that (the Flood) the sons of God (fallen angels) had the sexual relationship with the daughters of men, because after the Flood in the earth had lived the giants as the Bible also proves this. The fallen angels impregnated satanic seed to the daughters of men, and as the consequence of this their descendants (the giants) made filthy and satanic sins.
You can ask that how it is possible, that after the Flood can be the giants, for in the Ark were only Noah and his family no other people. The fallen angels are evil spirits, which after the Flood impregnated again the daughters of men. The fallen angels are not limited and bound by the borders of the human body, because they are evil spirits and the fallen angels didn't die on the Flood, but they continued deceiving by sins the men after the Flood. Because the Bible speaks that after the Flood, were giants and their descendants in the earth, it means that the fallen angels impregnated again the daughters of men. Their descendants became giant nations, in which the Old Testament tells.
Before the Flood, the fallen angels and the daughters of men had been the sexual relationship with each other, and for this reason evil of sin got terrible proportions in the earth. According to the Bible, God commanded to destroy the nations, who were after the flood descended from fallen angels and the nations which had adopted their habits. Evil spirits (fallen angels) had the sexual relationship with the daughters of men, and their descendants were under evil sins like burning their children in the fire as offerings to the idols. They lived violently, contacted with evil spirits, practiced unnatural sexual activities (homosexuality, bestiality).
These nations were not willing to repent of their sins and wanted to spread evil sins throughout the world, and they were completely controlled by satanic powers, which is why they were doing satanic deeds (burned their own children). For this reason God allowed to destroy these nations, because otherwise evil sins would have destroyed the whole world.
If God would not have allowed to destroy these satanic nations, so it could be that today in the world someone would burn little children as offerings to idols. I can understand at this point the Bible and God, and I would see injustice and unjust, if in the world would be people who would kill their own children, burning them with fire.
God commanded to destroy descendants of the fallen angels and the daughters of men, and the nations that had adopted their satanic sins. This command was given at a certain time and concern to certain nations, not all nations. During the New Covenant is not this kind of command.
Many people say that they don't understand the command of God to destroy those nations, who descended from fallen angels, but to accuse God for that. If God had not destroyed those nations, so now in the world should be nations, which do a lot of evil things and burned their own children to idols (evil spirits) as offerings.If God had not destroyed these nations, so the same people who accuse God for destruction of these nations, so those people would accuse today God that why He didn't destroy those awful nations which burn their own children in the fire. They would have said that, how does God allow such evil activity and doesn't do anything? However, God did properly when He destroyed these nations, regardless of whether even some people accuse on Him for that.