Pre-flood culture had undergone a thorough demonization. Fallen angels (demons) had taken over the souls and bodies of men and through marriage had produced Nephilim (fallen ones) who became the violent “mighty men who were of old, men of renown” (Ge. 6:4). Marriage had been demonized, and violence was idolized. Sexual violence was de jure. Therefore God decided to wipe the world clean of every trace of humanity, except for the man Noah.
Hughes, R. Kent. Genesis: Beginning and Blessing. Wheaton, IL: Crossway Books, 2004. Print. Preaching the Word.