1 Enoch 6:2
1 And it came to pass when the children of men had multiplied that in those days were born unto them beautiful and comely daughters. 2 And the angels, the children of the heaven, saw and lusted after them, and said to one another: 'Come, let us choose us wives from among the children of men and beget us children.' 3 And Semjâzâ, who was their leader, said unto them: 'I fear ye will not indeed agree to do this deed, and I alone shall have to pay the penalty of a great sin.' 4 And they all answered him and said: 'Let us all swear an oath, and all bind ourselves by mutual imprecations not to abandon this plan but to do this thing.' 5 Then sware they all together and bound themselves by mutual imprecations upon it.
Philo on the Giants 4
If, therefore, you consider that souls, and demons, and angels are things differing indeed in name, but not identical in reality, you will then be able to discard that most heavy burden, superstition. But as men in general speak of good and evil demons, and in like manner of good and evil souls, so also do they speak of angels, looking upon some as worthy of a good appellation, and calling them ambassadors of man to God, and of God to man, and sacred and holy on account of this blameless and most excellent office; others, again, you will not err if you look upon as unholy and unworthy of any address. And the expression used by the writer of the psalm, in the following verse, testifies to the truth of my assertion, for he says, "He sent upon them the fury of His wrath, anger, and rage, and affliction, and he sent evil angels among Them." These are the wicked who, assuming the name of angels, not being acquainted with the daughters of right reason, that is with the sciences and the virtues, but which pursue the mortal descendants of mortal men, that is the pleasures, which can confer no genuine beauty, which is perceived by the intellect alone, but only a bastard sort of elegance of form, by means of which the outward sense is beguiled; and they do not all take all the daughters in marriage, but some of them have selected some of that innumerable company to be their wives; some choosing them by the sight, and others by the ear, others again being influenced by the sense of taste, or by the belly, and some even by the pleasures below the belly; many also have laid hold of those the abode of which is fixed at a great distance, putting in action various desires among one another. For, of necessity, the choices of all the various pleasures are various, since different pleasures are established in different places.
Notes and References
"... The problem of evil and human suffering was apparently an important and popular topic for writers in Judaism during the closing centuries B.C.E. and the early first century C.E. Recent discoveries amongst the Dead Sea Scrolls attest that the people of the Qumran Community were concerned why the people of Israel, and perhaps humanity in general, were suffering. One such explanation for human suffering is detailed in the Watcher tradition that is found in 1 Enoch 1-36, the Book of Watchers. The Watcher tradition of 1 Enoch, considered by most scholars as an expansion of Genesis 6:1-4, tells the story of the origin of evil spirits. These spirits were a result of the sexual union of the bene elohim and human women, which produced giant offspring. Furthermore, the giant tradition of BW was taken up by several other Second Temple Period Jewish writings and further developed alongside an anthropology which assumed a human nature that is weak, corruptible, and subject lo the manipulation of spiritual forces. However, this was not the only Jewish understanding of the problem of evil ... In what follows, I will present an alternative approach to the responsibility for human suffering articulated by a lone voice in the Alexandrian Diaspora during the first century C.E., Philo of Alexandria. Philo's understanding of human suffering differs considerably from the tradition set forth in the Watcher tradition ... it is difficult to determine whether Philo knew of the Watcher tradition as we have it in but it appears that he was aware of some form of the Fallen Angel tradition prevalent in Palestinian Judaism ..."
Wright, Archie Some Observations of Philo's De Gigantibus and Evil Spirits in Second Temple Judaism (pp. 471-488) Journal for the Study of Judaism, 2005