We read about sons of God and daughters of men first in the Holy Bible in (Genesis 6:2) In the 4th. Verse we read that there were giants in those days and by the union of sons of God with daughters of men they begot mighty men which were of old. Here many Bible readers confused themselves as the giants were born due to the union of sons of God with daughters of men, but as per (Gen. 6: 4) it was wrong, and giants were there before the sons of God united with the daughters of men! Also it is recorded that by the union of the sons of God with the daughters of men there arose mighty men which were of old, men of renown. As per (Gen. 6:3) God was not pleased on the sons of God marrying daughters of men and He had reduced their life to 120 years, though other men lived longer, as from Adam to Noah people lived longer and Noah lived 950 years. So though people in that period lived long, God had reduced the life of them who were born to sons of God and daughters of men to 120 years, though they were mighty and renown. These mighty people might be wicked using their strength towards violence. But the wicked nature of the people made God to repent for creating man and He decided to destroy them by flood.
So, we understand that the giants were there prior to the union of sons of God with the daughters of men. Secondly we need to understand who are these sons of God? In (Gen. 4:26) we read that after the birth of Enos to Seth (Adam’s third son), men of those days began to call on the name of the LORD. So we may call those men as the sons of God as they called upon the name of the LORD. But the other sons of Adam like cain, did not call on the name of the LORD. So we may call them as sons of men, and the women as daughters of men.
Secondly we read about the sons of god in (Job 1:6, 2:1) Here the sons of God came together to present themselves before the Lord, and satan also came there. Now let us find the place where the sons of God came together? It may be on the earth or it may be in heaven. But we can rule out heaven as satan cannot go to heaven. So we are sure that the sons of God came together on the earth only. Now who are these sons of God? Are they men or angels? Because they came together on the earth, they must be men or devoted men came to worship God in His presence where they gathered, and God also left heaven and went to earth to receive the service of men and to bless them. So we can be sure that they were not angels. Here satan went along with those men to God’s presence, not to worship but to observe and if possible to provoke any man or few men to do evil. As God saw the devil, he admired the qualities of the righteous Job. So we are sure that these sons of God are men only.
Thirdly in (Job 38:4-7) we read about the sons of God, Shouted for joy during the creation of the earth, while the morning stars sang together. Here certainly the sons of God and the morning stars were not men but must be angels, as Adam was created only after the re- creation of the heaven and the earth in seven days (6+1 days), which was after the first creation of heaven and earth by God (Gen. 1:1). Also in (Judges 5:20) we read about the stars from heaven fought against Sisera, the captain of Jabin the king of Canaan. So we understand that these stars were angels.
Now let us ponder the qualities of the angels. They were created by God before man was created and they were in heaven serving God. The Cherubims are the vehicles for God and God travels using them. (Ezek. 10:18) (Psa. 18: 10) In (Ezek.28: 14) we read about Cherub created by God to protect or to cover. In (Psa. 78; 49) we read about the evil angels sent upon Egyptians during the time of Moses. In (Heb. 1: 7, 14) we read about the angels as ministering spirits sent to minister the heirs of salvation. In (Joshua 5: 14) we read about an angel who was the captain of the host of the Lord who helped Joshua to conquer Jericho city. In (Luke 24: 39) we read about the truth that the spirit does not have flesh and bone, or spirit cannot be touched or handled. In (Luke 20; 35, 36) we read about angels will not marry at all. We can read more about the angels in the Holy Bible. They do many duties but the basic nature of angels is they are spirits which cannot be touched. So the angels cannot marry daughters of men and produce children at all.
By combining all the three points we can come to a conclusion that angels did not marry any daughter of man, but the godly generation like children of Enos married daughters of Cain who were ungodly but carnal. Perhaps they might had decorated themselves to gain artificial beauty and the godly generation might be deceived by their appearance and their generation might be mighty but not giants. But God reduced their age to 120 years as God was not pleased in such union, though they became mighty and renown. For example we read about (Gen. 36: 24) Anah, the son of Zibeon who found mules through asses as he reared his father’s asses, and these mules are stronger than asses and horses in carrying heavy loads. Only in the 18th generation people lived 120 years and Moses was one among them. But after the flood only the generation of Noah was existing who was the generation of Enos the son of Seth and the children of Cain or any other sons of Adam were not there as they were destroyed in the flood.