Genesis 6:1-4 refers to the sons of God and the daughters of men. There have been various suggestions as to who the sons of God were and why the sons they had with the daughters of men became a race of giants (that is what the word Nephilim seems to indicate).
The three main views on the identity of the sons of God are 1) they were fallen angels, 2) they were powerful human rulers, or 3) they were godly descendants of Seth who married wicked descendants of Cain. The first theory is given weight by the fact that in the Old Testament the phrase “sons of God” always refers to angels ( Job 1:6 ; 2:1 ; 38:7 ). A potential problem with this is in Matthew 22:30, which indicates that angels do not marry. The Bible gives us no reason to believe that angels have gender or that they can reproduce. The other two views do not have this problem.
The weakness of views 2) and 3) is that ordinary human men marrying ordinary human women does not explain why the descendants were “giants” or “heroes of old, men of renown.” Also, why would God decide to bring the flood on the earth ( Genesis 6:5-7 ) when God had never prohibited powerful human males or descendants of Seth from marrying ordinary human females or descendants of Cain? The coming judgment of Genesis 6:5-7 is related to what happened in Genesis 6:1-4. Only the obscene and perverse marriage of fallen angels with human women would seem to justify such a harsh judgment.
As noted above, the weakness of the first view is that Matthew 22:30 states: “In the resurrection people will not marry or be given in marriage; They will be like the angels in heaven.” However, the text does not say “angels cannot marry”. Rather, it just indicates that angels do not marry. Second, Matthew 22:30 refers to “angels in heaven.” It does not refer to the fallen angels, who do not care about God’s created order and actively seek ways to disrupt God’s plan. The fact that God’s holy angels do not marry or have sexual relations does not mean that the same is true for Satan and his demons.
View 1) is the most likely position. Yes, it is an interesting “contradiction” to say that angels are sexless and then say that the “sons of God” were fallen angels who procreated with human females. However, while angels are spirit beings (Hebrews 1:14), they can appear in physical human form (Mark 16:5). The men of Sodom and Gomorrah wanted to have sex with the two angels who were with Lot (Genesis 19:1-5). It is plausible that angels are capable of taking human form, even to the point of replicating human sexuality and possibly even reproduction. Why don’t fallen angels do this more often? It seems that God imprisoned the fallen angels who committed this evil sin, so that the other fallen angels would not do the same (as described in Jude 6). The early Hebrew interpreters and the apocryphal and pseudopigraphical writings are unanimous in holding the view that the fallen angels are the “sons of God” mentioned in Genesis 6:1-4. This by no means closes the debate. However, the view that Genesis 6:1-4 involves fallen angels mating with human women has a strong contextual, grammatical, and historical basis.