Who or what were the “giants in the earth in those days” (Gen 6:4)?

This is one of the most puzzling texts in the Bible. The KJV translation “giants” is from Septuagint translation, γίγαντες or gigantes, of the Hebrew word נְפִיל, nephilim. This word can also mean “the fallen [ones],” suggesting corrupt, wicked men, the word itself most likely from נָפַל, naphal, “to fall.” This is a very slender bit of linguistic evidence on which to build notions of demon-spawned giants, which is a common theory. The reason I personally find this theory unlikely is simply that there is no other evidence in the Bible suggesting either that demons can sire earthly offspring or that there were supernaturally large giants, and rarely if ever is there so slender evidence offered for such theologically consequential creatures. If the author wanted you to believe in demon-spawned giants, surely he would have said more. Besides, there are much more reasonable theories, associated with interpretations (2) and (3), above, of the “sons of God.” Here then is the summary case for another theory. Note first that Gen 6:1-8 comes at the end of two genealogies—not separated in the original by chapters—describing the corruption of Cain’s line and the comparative righteousness of Seth’s. These verses serve as a grim conclusion to two toledoth sections about men who, whether regardless how their family line began, mostly became wicked and all-too-mortal. Then we are told, by way of summarizing the outcome of it all, that men were fruitful and multiplied, and “the sons of Gods saw the daughters of men,” etc. These (so to speak) pregnant remarks, I propose, crucially advance the narrative by explaining how Seth’s line of Lord-callers and walkers-with-God were corrupted: basically, Seth’s line, dignified in the description as “sons of God,” did not abstain from marrying the beautiful but wicked women of Cain’s line. The implication of the verses is similar to the many instances of Israelites intermarrying with “strange women” and being corrupted thereby. Indeed, the offspring of such unions were “mighty men which were of old, men of renown” (6:4), but immediately we are told, “God saw that the wickedness of man was great in the earth” (6:5). One thinks of Gilgamesh and other kings who claimed divine blood (the epic says he was two-thirds god and one-third man); the wording in the text is winking at such myths, particularly considering that 6:4’s “and also after that,” i.e., after the Flood there would be similarly arrogant strongmen, supposedly demigods but really just corrupt and powerful men. Indeed, the text may suggest that all men were like the nephilim insofar as the more divine and righteous cultural features all but died due to intermarriage. So the cultural dominance of Cain’s line was total, and Noah’s family was therefore the last remnant of the Seth line.