Genesis 6:2

That the sons of God saw the daughters of men, that they
were fair
Or "good" ^F11, not in a moral but natural sense; goodly to look upon, of a beautiful aspect; and they looked upon, and only regarded their external beauty, and lusted after them: those "sons of God" were not angels either good or bad, as many have thought, since they are incorporeal beings, and cannot be affected with fleshly lusts, or marry and be given in marriage, or generate and be generated; nor the sons of judges, magistrates, and great personages, nor they themselves, as the Targums of Onkelos and Jonathan, and so Jarchi and Aben Ezra; but this could be no crime in them, to look upon and take in marriage such persons, though they were the daughters of the meaner sort; and supposing they acted a criminal part in looking at them, and lusting after them, and committing fornication with them, and even in marrying irreligious persons; yet this could only be a partial, not an universal corruption, as is after affirmed, though such examples must indeed have great influence upon the populace; but rather this is to be understood of the posterity of Seth, who from the times of Enos, when then began to be called by the name of the Lord, ( Genesis 4:25 ) had the title of the sons of God, in distinction from the children of men; these claimed the privilege of divine adoption, and professed to be born of God, and partakers of his grace, and pretended to worship him according to his will, so far as revealed to them, and to fear and serve and glorify him. According to the Arabic writers ^F12, immediately after the death of Adam the family of Seth was separated from the family of Cain; Seth took his sons and their wives to a high mountain (Hermon), on the top of which Adam was buried, and Cain and all his sons lived in the valley beneath, where Abel was slain; and they on the mountain obtained a name for holiness and purity, and were so near the angels that they could hear their voices and join their hymns with them; and they, their wives and their children, went by the common name of the sons of God: and now these were adjured, by Seth and by succeeding patriarchs, by no means to go down from the mountain and join the Cainites; but notwithstanding in the times of Jared some did go down, it seems; (See Gill on Genesis 5:20) and after that others, and at this time it became general; and being taken with the beauty of the daughters of Cain and his posterity, they did as follows: and they took them wives of all that they chose;
not by force, as Aben Ezra and Ben Gersom interpret, for the Cainites being more numerous and powerful than they, it can hardly be thought that the one would attempt it, or the other suffer it; but they intermarried with them, which the Cainites might not be averse unto; they took to them wives as they fancied, which were pleasing to the flesh, without regard to their moral and civil character, and without the advice and consent of their parents, and without consulting God and his will in the matter; or they took women as they pleased, and were to their liking, and committed fornication, to which the Cainites were addicted; for they spent their time in singing and dancing, and in uncleanness, whereby the posterity of Seth or sons of God were allured to come down and join them, and commit fornication with them, as the Arabic writers