Everyone that claim that the sons of God are fallen angels by using Job alway miss this part. Job 1:6 "Now there was a day when the sons of God came to present themselves before the Lord, and Satan also came among them. The context separates satan from the sons of God and Job 2:1 further separates him from the sons of God "Again there was a day when the sons of God came to present themselves before the Lord, and Satan also came among them to present himself before the Lord." By adding to present himself before the Lord.
Yes satan was with or among the sons of God when they came before the Lord, but was not counted as one of the sons of God. Why? Because he "also came among them." 2:1 adds, "to present himself before the Lord." again not part of the sons of God, only among the sons of God, but he was not a son of God. I know there are some that say that because it's in heaven that that proves that satan is one of the sons of God, but what does Revelation say, he is before God accusing the brothers both day and night, does that sound like what the sons of God were or would do?
Plus when Jesus spoke about the resurrection in Matthew 22:30 "For in the resurrection they neither marry nor are given in marriage, but are like angels in heaven." Jesus said angels do not marry and from what I understand angel is used in the neuter form (I don't know for sure) and it is clear that the sons of God in Genesis 6:2 married the daughters of men, "and they took wives for themselves of all whom they chose. "So from a Biblical viewpoint, they were not committing fornication, sex with someone that is not your husband or wife is fornication or illicit sex.
Look at 6:5-7 the Lord states the reason He is going to judge the earth, “Then the Lord saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually. 6 And the Lord was sorry that He had made man on the earth, and He was grieved in His heart. 7 So the Lord said, “I will destroy man whom I have created from the face of the earth, both man and beast, creeping thing and birds of the air, for I am sorry that I have made them.” Notice how the Holy Spirit guided Moses to used the word "man" instead of men, the plural form, I wonder if it's because He used men to separate them from the son of God and now by using man He is bring all humans together for the judgement? Something to think about.
No mention of the union/marriage of the sons of God to the daughter of men, but if was because of mans heart was intent, thoughts, only evil always. Where in the Bible does it say that a being that has rebelled against God is one of His child or son(s)?
Please do not bring up the book of Enoch, because in 10:12 we know that 10 talks about children of fornication by the watchers or angels and some believe this is talking about Genesis 6, when it's not. Enoch 10:10-13 "Restore the earth, which the angels have corrupted; and announce life to it, that I may revive it. 11. All the sons of men shall not perish in consequence of every secret, by which the Watchers have destroyed, and which they have taught, their offspring. 12. All the earth has been corrupted by the effects of the teaching of Azazyel. To him therefore ascribe the whole crime.13. To Gabriel also the Lord said, Go to the biters, to the reprobates, to the children of fornication; and destroy the children of fornication, the offspring of the Watchers, from among men; bring them forth, and excite them one against another. Let them perish by mutual slaughter; for length of days shall not be theirs."
This is not talking about Genesis 6, the Lord did not say "restore the earth" He said he was going to destroy the men of the earth, not because of the children of the sons of God and the daughters of men. He was going to destroy men because they were evil in thoughts in their heart continually and He judged man, not azazyel. The children from the union again were not of fornication, because the sons of God and the daughters of men were married, so no fornication there or children go fornication.
Who were the sons of God? Genesis 4:25-26 "And Adam knew his wife again, and she bore a son and named him Seth, “For God has appointed another seed for me instead of Abel, whom Cain killed.” 26 And as for Seth, to him also a son was born; and he named him Enosh. Then men began to call on the name of the Lord." This is why, some believe think they are the descendants of Seth, because Seth is another seed, instead of Abel. Remember, God said "between your seed and her seed"? Abel was the seed that was going to bring forth the Messiah. This is why the Holy Spirit had Moses write it down like that, so we could know that "her seed" was still going to be at enmity with "his seed". And his son Enoch was born and men began to call on the name of the Lord.
Who are the daughters of men, Genesis 4:16-17 "Then Cain went out from the presence of the Lord and dwelt in the land of Nod on the east of Eden. 17 And Cain knew his wife, and she conceived and bore Enoch. And he built a city, and called the name of the city after the name of his son—Enoch." Since Cain went out from the presents of the Lord, which has to mean that he did not follow the Lord or his heart had not changed, he still had the heart of fruits of the ground for an offering to the Lord. Because we know that you can not go from the presenceof the Lord, Psalms 139:7-12.
This is how people come to the conclusion that the daughters of men, were Cain's offspring, because Cain's clan live apart from Seth's offspring and with Seth's son Enosh men began to call on the name of the Lord. So they were living different types of lifestyles. The main thing to remember of why the Lord wanted to stop this is because of Eve's seed or the promised Messiah and Seth was another seed after Abel's death to take Abel's place in that line to bring about "her seed", because it says "and another seed" that was Eve's reaction to Seth's brith.
Hope that helped, I know no one is going to change their view, but for those that aren't sure, that's what is behind the Seth and Cain belief.