Jubilees 7:20
20 During the twenty-eighth jubilee [1324-72] Noah began to prescribe for his grandsons the ordinances and the commandments — every statute which he knew. He testified to his sons that they should do what is right, cover the shame of their bodies, bless the one who had created them, honor father and mother, love one another, and keep themselves from fornication, uncleanness, and from all injustice. 21 For it was on account of these three things that the flood was on the earth, since it was due to fornication that the Watchers had illicit intercourse — apart from the mandate of their authority — with women. When they married of them whomever they chose, they committed the first acts of uncleanness. 22 They fathered as their sons the Nephilim. They were all dissimilar from one another and would devour one another: the giant killed the Naphil; the Naphil killed the Elyo; the Elyo mankind; and people their fellows.
Pseudo Jonathan Genesis 6:2
1 When the children of men began to multiply on the face of the earth, and beautiful daughters were born to them. 2 The sons of the great ones saw that the daughters of men were beautiful, that they painted their eyes and put on rouge, and walked about with naked flesh. They conceived lustful thoughts, and they took wives to themselves from among all who pleased them. 3 The Lord said in his Memra, 'None of the evil generations that are to arise (in the future) will be judged according to the order of judgment applied to the generation of the Flood, (that is) to be destroyed and wiped out from the world. Did I not put my holy spirit in them that they might perform good deeds? But behold, their deeds are evil. Behold, I gave them an extension of a hundred and twenty years that they might repent, but they have not done so.'
Notes and References
"... The concluding reference in passage A to the nudity of the Generation of the Flood, appears to be an adaptation of a much older tradition relating to the Antediluvians' immoral practices. One of the principal offences of the Generation of the Flood, according to R. Meir and R. Judah the Patriarch, was the public display of their nudity, (See particularly Targum Pseudo-Jonathan to Genesis 6:2 (from its inclusion in a Targum, we may presume that this tradition presumably gained wide currency in Talmudic times) alluding, no doubt, to the practices of the Graeco-Roman world. In the following passage, this tradition, which may have its origins in pre-Christian times, (See Jubilees 7:20, where Noah is depicted as admonishing his children to avoid the practice of uncovering the flesh) is associated appropriately with Job 24:7 ..."
Jacobs, Irving The Midrashic Process: Tradition and Interpretation in Rabbinic Judaism (p. 31) Cambridge University Press, 1995