In Genesis 6, the angels took wives. In my footnotes, it says the Sons of God were angels. What's this all about?
The divine beings in Gen 6:1–4 are called “sons of God,” which in this case likely means “divine beings,” i.e., those who belong to the category of divinity. The word “son” can carry this general sense in biblical Hebrew (and other ancient Semitic languages); it is not limited to biological male offspring. (In this instance, however, it is clear that what is intended is divine beings of male sex.) The text does not specify that they are angels; neither do they function as divine messengers (which is what the word “angel” connotes).
The claim of the story is that human women and male divine beings had sexual relations, which resulted in semi-divine offspring (called here “Nephilim,” who appear elsewhere in Num 13:33 as giants). There are a number of instances of such co-mingling of the human and the divine in the ancient Near East, including in the Bible. The Mesopotamian royal figure Gilgamesh is said to have been two-thirds god and one-third human. The biblical figure Samson is born to a human mother but is apparently fathered by a divine being (Judges 13). And, of course, the Christian traditions of the birth of Jesus recount that he had a human mother and a divine father. Another Christian reflex of this tradition is found in the Pauline admonishment to women to cover their heads “because of the angels” (1 Cor 11:10).