There is a problem with what you said. Mainly Noah was the only good man alive at this time. So when all of the families were wicked save one man, where did the good families come from?
Both the fallen angels interpretation and the both sides being human interpretation imply that it took time (probably generations) for the amount of wicked people and for the culture of wickedness to grow to the point that the flood became necessary. At bare minimum, it would have been 120 years, which itself is plenty of time (pre-flood, the earliest recorded age of a person who was born becoming a parent is 65, and it could have happened earlier than that).
Let's look at the implications of the fallen angels interpretation. This interpretation as I understand it (as numerous Christians have told it) is as follows:
1) Fallen angels decide to take human women as wives. These wives bear them children. These children are powerful giants, Anakites.
2) These powerful giants are also thoroughly wicked. The idea being to taint all of humanity with this bloodline to make the coming of Christ, and the redemption of humanity, impossible.
3) They make good enough progress so that God needs to start a global flood to wipe them out. Somehow, Noah doesn't have any of this demon-blood in him, and somehow, they didn't kill him, in spite of their evil purpose.
4) The flood happens, wiping them out. God learns his lesson and puts a limit on demonic activity, preventing them from doing this again. He throws all the demons who did this in prison, leaving the rest on Earth to tempt, deceive and possess people.
However, there are some serious problems with the implications of this story.
- In Numbers 13, the Anakites are back. Did God not do the job right the first time? Did he restrain some demons from sexual activity, but not others? Did one slip through? Did one or a few escape for a short bit to renew the Anakite race, in contradiction of this tale's interpretation of 1 Peter 3:19?
- Why is Satan still loose, if he was the #1 fallen angel, as Revelation 12:7-12 states? And if he was still loose and if this were possible, why not just do this all over again? Or did these fallen angels have a different ringleader? If so, who? Which of the fallen angels would be more wicked, clever, and decisive than Satan himself (this would be necessary because creating a race of demon-children seems a lot more effective than just tempting people)? And if it really was Satan who did it, why did God give him enough free reign to do this in the first place only to restrict it later? And why is he not in prison as the other fallen angels who did this are said to be? Why leave him free to tempt people to make large-scale wars, commit genocide, mass rapes (where no demon-blood is involved), institute religions that require child sacrifice, and institute slavery?
- More to the point, if all this were possible, why wouldn't Satan himself, or any of these demons, just overpower Adam, and rape Eve after their banishment?
So the fallen angels interpretation, and its accompanying story, paint God as being less clever or less powerful, Satan as being less clever or less powerful, and his rank-and-file demons as being more clever and more powerful than the Bible seems to teach. You pointed out one problem that isn't really a problem with my interpretation (Noah being the one righteous head of a family left on Earth). I see many more problems with this tale, which are not consistent with what Scripture teaches about God, Satan, and the demons. If the "sons of God" refer to good men, some say from the line of Seth exclusively, then none of the problems I brought up exist. We have a narrative that is consistent with the Law's rebuke to not marry foreign women, and the story of foreign women corrupting Israel's kings. It just as adequately explains the need for the flood, it serves as a greater warning to us, showing us how far plain old people can fall without any genetic interference from demons. It has Nephilim as a term for great and wicked
homo sapiens sapiens, which also resolves the issue of the Anakites coming up again in Numbers 13, and presents no potential issues for the redemption of the descendants of their surviving offspring.