Note: All Scripture quotations found in this article are from the text of the Authorized King James Version of the Bible.
This will be a brief article proving that the sons of God in the Old Testament are the fallen angels and not the "sons of Seth/men".
Genesis 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."
Genesis 6:4 "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."
Notice the distinction between the sons of GOD and daughters of MEN. It is clear these are angels because if they were just men, there would be no distinction. These angels are the angels that "kept not their first estate"(Jude 1:6). Also, notice their offspring were giants.
Job 1:6 "Now there was a day when the sons of God came to present themselves before the LORD, and Satan came also among them."
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."
These are the fallen angels because they are with Satan, who had also fallen, but remember that Satan is a Cherub (Ezekiel 28:14) and not an angel. He only appears as an angel of light (2 Cor. 11:14).
Job 38:7 "When the morning stars sang together, and all the sons of God shouted for joy?"
If you read in Job 38:4, you will see that this event was around the time God laid the foundations of the earth, which was before Adam or Seth was even around. The morning stars are angels like in Revelation 1:20. The sons of God are with the angels in Heaven singing and shouting. These sons of God are clearly not human, rather the angels.
Later on in the New Testament, the people that are saved are called the sons of God (Romans 8:14, 19; Philippians 2:15, 1 John 3:1) because we are to replace the fallen angels that fell and even help Jesus Christ judge them at the White Throne Judgment (1 Cor. 6:3). What is even more amazing is that when we are Raptured, our bodies will be glorified and fashioned like His body. Keep in mind that every angel in the Bible is a male, Jesus Christ was male and Adam who was made in the image of God was male (Genesis 1:26). Also, there will be no marriage in heaven (Mark 12:25).
1 John 3:2 "Beloved, now are we the sons of God, and it doth not yet appear what we shall be: but we know that, when he shall appear, we shall be like him; for we shall see him as he is."