The sons of God have generally been understood as fallen angels (or demons). The main interpretation of the phrase seems to refer to demons. The same Hebrew phrase is in Job 1:6; 2:1; 38:7; Psalms 29:1; 89:7. This idea appears often in ancient Jewish and Christian writings (for example, 1 Enoch 6:1–7:6; Justin Martyr, Apology 2.5). The New Testament also seems to support this view in some passages (see 1 Peter 3:18–20; 2 Peter 2:4; Jude 1:6–7).
Some interpreters argue that God would not allow angels to have children with humans. They doubt that the New Testament texts should be understood in this way.
Another possibility is that the phrase the sons of God refers to the righteous descendants of Seth. The beautiful women would be from Cain's evil line. This matches Genesis 4:17–5:32 but is weakened by the language of 6:1–2. That passage seems to refer to all human daughters, not just Cain's.
Others believe the phrase the sons of God refers to cruel human kings. These kings were possibly influenced by demons. They followed Lamech’s practice of having multiple wives and made it even worse (4:19) by seizing the daughters of righteous people. The language is similar to 3:6 ("saw … took"). It shows the rebellious nature of their actions.