What you are discussing is not a story that originated with the Hebrews. It is part of the Mesopotamian cosmology that, apparently, the Hebrews felt compelled to include in this section of the Torah.
It is also the third (and likely the most disbelieved among the Hebrews) creation story in Genesis.
Chapters 1-10 of Genesis are a preface to the Torah. They include a mythology based on legends and Mesopotamian cosmology before entering the “historic” phase of Torah, notably the story of Abraham. Before Abraham we are not reading history in Genesis.
What is “cosmology”?
Cosmology is basically the science that deals with explaining the universe. In ancient Mesopotamia, this was combined with the common, everyday philosophical and religious views (often thought of as one single view), creating a “cosmology” that wouldn’t be allowed under today’s strict critical methodologies. Of course, those methods had not yet been devised in the ancient world, so we can't demand that when we read their legends. And while the Hebrews had their own ideas on the universe, they merely interpolated they ideas into the “science of the day” apparently to make it more palatable to the ancient reader. In other words, the Jews were giving their religious explanation of the Mesopotamian cosmology which itself was shaped by religious views of the heathen world (again, science, religion, and philosophy all being considered as “one” back then).
The Angels, the Nephilim, and the Flood: Not Much Liked by the Jews
The main cosmology of ancient Mesopotamia was that the visible universe was created from a vast cosmic sea of water. Instead of imagining outer space as being a vacuum, the ancient Mesopotamians believed it was an ocean of sea water.
The water had to be held back by a metallic dome upon which the luminaries of the stars, moon, and sun were attached. There were doors in the dome that would open from time to time to allow the cosmic water to seep down as precipitation. Earth was a plate of land supported upon pillars that extended deep into the cosmic ocean of space (also known as the “abyss”).
According to this cosmology, the present universe was the second attempt by the gods to create a world of humanity. The present world was developed from the best of the previous world, which was wicked and evil. The gods either got bored (or angry, depending on the actual cosmological philosophy you subscribed to) with the wicked people (or sometimes with other gods) resulting in a universal cataclysm. The best of the previous world was saved by the most pure or righteous man on the planet who placed all that was worthy in a vessel (sometimes provided by the deities) and thus preserved earthly life for the new world that followed.
The Jews didn’t particularly like this story. If you notice, though it is the primary creation story of Mesopotamian culture, it is placed third and last in the pre-history section of Genesis. The first two creation stories are Genesis chapter 2 (which is the oldest) and Genesis chapter 1 (which was invented around the time of the Babylonian exile). There is a grand separation between these two and the Flood-Creation paradigm of the Mesopotamians.
So everything you read here in this story, the angels that chose wives, the resulting wickedness, and the Flood, all that is not Jewish in origin. Noah and the Nephilim, however is of Jewish origin.
“We Didn’t Hate or Like Your Story, So We Edited It Down and Made Our Own Character the Hero.”
If you are a screenwriter for the major film industry, you hear this alot. Ever read the credits when a movie begins and it says something like “written by So-and-so and So-and-so”? Then there’s a space between these names and it gets followed by either “and by So-and-so” or (worse) “based on a story by So-and-so.” Usually if you are just starting out in the business as a scriptwriter, you tend to be the last “So-and-so” for a while. This means the studio purchased your story, but then they ripped it apart and gave it to other more experienced writers to change it into what it finally became. You may have been the original writer, but the studio needed to make some fast cash with another action or romance flick (which your original script was not).
Simply put: you’ve been screwed over as the patsy, your character and other story elements have been taken, and what may have been a drama about a person in very normal circumstances overcoming a challenge is now a cowboy-space epic about a four-breasted alien who overcomes evil with her powerful diamond cannon blaster and her cute blobby sidekick named Blagh. Your original story is, uh, kinda still in there...somewhere.
This is sort of what the Jews thought of the Flood/Creation cosmology of the Mesopotamian culture...and what they did with it. It was the “science” of the day, and they had already incorporated the whole “dome things holding back the waters” in Genesis 1. So when before they ended the “origins” or mythology section of Genesis, they decided to take a crack at explaining their take on what the rest of the world believed was true.
Note how little room is given to explaining what caused the flood: just 8 verses. In these 8 verses, humanity increases, divine beings take wives from among the humans, God decided to shorten the lives of people, giants are born, and then God says the world is wicked and needs to be destroyed.
Yeah, it’s disjointed. None of the things actually say that something evil was going on. There are no laws forbidding this. The “divine beings” that marry women are likely angels, but the text doesn’t leave out the possibility that they were just humans who had this status of being “sons of God.” Just seven chapters earlier God pronounced everything good, but all of a sudden it went bad? And who the heck are the Nephilim?
Well, that shows you what the Jews thought of all this...very little. The Jewish Study Bible explains:
This brief narrative reads like a condensation of a much longer, well-known myth….It also explains the origin of the Nephilim (v. 4), the preternatural giants that Israelites thought once dwelt in the land (Num 13.31-33).
Not only is this not a Jewish myth, the Jews use this opportunity to insert a reason for our own legends about the ‘giant people’ that once inhabited the Fertile Crescent. If they had existed, this is where they came from.
The idea that God is angry with humanity and decides to “flood” the world to destroy it is later contrasted with the end of the catastrophe where God suddenly uses this opportunity to bind God’s own self to a law preventing God from ever doing the same thing again. (Genesis 9:8-17) This appears to be an attempt at mocking the Flood/Creation cosmology.
The gods of the Mesopotamian Flood/Creation stories do not repent of what they did in flooding the world. This contrasts with the God of the Hebrews who is binded by an everlasting law contract from doing such a thing. In other words, the Jews are calling their neighbors silly, essentially saying: “You worship a god who will destroy you at a mere whim, whereas our God is bound by Divine Law from ever doing so!”
While Noah has the status of a genuine person in Hebrew history, the actual flood as described in the Bible does not. Some academics have suggested that the Jews had a story of Noah surviving a cataclysmic flood with his family and a few farm animals on a raft, and that this story may have been interpolated into the Mesopotamian Flood/Creation paradigm. While this is not actual Jewish theology, the concept seems likely.
In the end, the story is not as the Jehovah’s Witnesses present it. Angels were not "sexually attracted to women," not really.