The last remaining New Testament passage which seems to indicate that strange things were occurring around the time of Noah comes from the book of Jude (emphasis added):
Now I desire to remind you, though you know all things once for all, that the Lord, after saving a people out of the land of Egypt, subsequently destroyed those who did not believe. And angels who did not keep their own domain, but abandoned their proper abode, He has kept in eternal bonds under darkness for the judgment of the great day, just as Sodom and Gomorrah and the cities around them, since they in the same way as these indulged in gross immorality and went after strange flesh, are exhibited as an example in undergoing the punishment of eternal fire. (Jude 5-7)The first thing to notice about this passage is that it does not list the events in chronological order. Although Jude lists the exodus from Egypt first, this event occurred after Sodom and Gomorrah. The key part of this passage which supports the conclusion that the sons of God are angels is the phrase:
in the same wayIn the same way that Sodom and Gomorrah indulged in gross immorality and went after strange flesh, so the angels also did the same. Out of all the passages in the New Testament, this passage provides the strongest support for some sort of angelic immorality. Since it doesn't seem like Jude would simply pull angelic immorality out of nowhere, and since Genesis 6:1-4 is the only passage in the Old Testament that hints at angelic immorality, then it seems likely that Jude was referring to the sons of God in Genesis 6.
How exactly did the angels in Noah's time indulge in immorality and go after strange flesh? There are some who believe that they mated with the daughters of men and conceived demon babies— babies that were half human and half demon. From this it is argued that God sent the flood because these demon babies were unredeemable (because of the cross Jesus can pay for the sins of men, but His sacrifice on the cross cannot pay for the sins of angels— therefore, since these demon babies were part demon, they could not be saved). However, such a view regarding demon babies is nothing more than a fictional fantasy. Genesis 6 is very clear that the reason for the flood was man's wickedness (...not the creation of a new breed of creatures).
The better interpretation is that the sons of God possessed either the daughters of men or their male mates (...or both). Demons were capable of possession from the very beginning (as is evident from Genesis 3 where Satan possessed the body of a serpent). Through the means of possession, demons were somehow able to carry out their desires, but the idea that these demons could pro-create without possessing a human body is simply illogical.
Who Are the Sons of God? (Part 6) - Genesis 6:1-4
State of the World (2948 BC): Noah's Birth - Genesis 5:28
The Fall - The Christian Worldview
Summary of Genesis 4
The Genealogy: Adam to Noah - Genesis 5