Now the giants were upon the earth in those days; and after that when the sons of God were wont to go in to the daughters of men, they bore children to them, those were the giants of old, the men of renown.
Genesis 6:4, Septuagint
And Uriel said to me: ‘Here shall stand the angels who have connected themselves with women, and their spirits assuming many different forms are defiling mankind and shall lead them astray into sacrificing to demons as gods, (here shall they stand,) till the day of the great judgement in which they shall be judged till they are made an end of.
Enoch 19:1, R. H. Charles
And the women also of the angels who went astray shall become sirens.’