Genesis 6:1, 2, “When man began to multiply on the face of the land and daughters were born to them, the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose.” ESV
“Three main theories have been suggested to identify the ‘sons of God’ and the ‘daughters of man.’ The first theory is that the ‘sons of God’ are fallen angels and the ‘daughters of man’ are mortals. The wickedness for which they are condemned is the unlawful marriage between those who are immortal and those who are mortal. This viewpoint hinges in part on the assumption that 2 Peter 2:4 and Jude 6, 7 refer to these angels. Proponents of the view point to the fact that the Hebrew term, ‘sons of God’ refers elsewhere to angels (Job 1:6, 2:1, 38:7). Jesus said in Matthew 22:30, however, that angels do not marry. Furthermore, God created all things to reproduce ‘according to their kinds’ (Genesis 1:24), which would seem to exclude the possibility of cohabitation between angels and human beings.
A second theory is that the ‘sons of God’ were godly descendants of Seth (Genesis 5:3-32) who married ungodly women from the line of Cain (Genesis 4:17-24). Thus the sin with which they are charged is one that is common throughout Scripture, and especially in the Pentateuch: the intermarriage of Godly people and those who are unholy. One difficulty with this view is that the term ‘sons of God’ is not used in this sense elsewhere in the Scriptures. Furthermore, with the exception of Noah (vv. 8,9), the men of that era could not be labeled as godly (v. 5).
A third theory is increasing in popularity. Kings were often associated with gods in the ancient world, and recent archaeological evidence has suggested that the phrase translated ‘sons of God’ may have been used to describe human rulers (although the Scriptures never ascribe deity to them). Therefore the ‘sons of God’ may refer to immoral human kings who used their power to take many wives.” ESV Hebrew-Greek Key Word Study Bible Notes
The Bible is full of mysteries. This one has three theories of explanation. Egyptian kings were considered ‘gods.’ It would not surprise me if the third theory was accurate.