The passage speaks of men (people), their daughters, and the "sons of God." If we include verses 3-7 in the passage, it suggests that the result of the marriages of the "sons of God" to the "daughters of men" was moral corruption of such extent and magnitude that God sent the Flood to wipe out humanity.
Commentators are divided in their interpretation of the passage. Some hold that the "sons of God" were the godly line of Seth, and the "daughters of men" were the offspring of Cain. In my opinion, there is evidence for this view. More easily defended is the seemingly bizarre interpretation that the "sons of God" were fallen angels or, at best, demon-possessed men bent on corrupting the human race. I believe that the New Testament refers to the incident in two places: 2 Peter 2:4 and Jude 6.
Not all fallen angels were cast into hell; not all are "into gloomy dungeons to be held for judgment." Many fallen angels are relatively free to roam about as Satan's servants. It may be that those who were cast into hell were guilty of another crime in addition to their original rebellion. They are the "sons of God" who tried to destroy the human race.