I ran across
this quote from Josephus, Antiquities of the Jews, 3:1. He held the common view that Genesis 6 talks about fallen angels having sex with mortal women:
1. NOW this posterity of Seth continued to esteem God as the Lord of the universe, and to have an entire regard to virtue, for seven generations; but in process of time they were perverted, and forsook the practices of their forefathers; and did neither pay those honors to God which were appointed them, nor had they any concern to do justice towards men. But for what degree of zeal they had formerly shown for virtue, they now showed by their actions a double degree of wickedness, whereby they made God to be their enemy. For many angels (11) of God accompanied with women, and begat sons that proved unjust, and despisers of all that was good, on account of the confidence they had in their own strength; for the tradition is, that these men did what resembled the acts of those whom the Grecians call giants. But Noah was very uneasy at what they did; and being displeased at their conduct, persuaded them to change their dispositions and their acts for the better: but seeing they did not yield to him, but were slaves to their wicked pleasures, he was afraid they would kill him, together with his wife and children, and those they had married; so he departed out of that land.
The footnote 11 says that "This notion, that the fallen angels were, in some sense, the fathers of the old giants, was the constant opinion of antiquity."
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