Canaanite literature used this same phrase to designate divine beings, members of the divine pantheon. This use of the phrase has been read into the passage by a number of modern scholars. They use ancient mythology to interpret the text because, supposedly, this passage contains ancient myths.
Extrabiblical materials can be helpful in understanding difficult passages.
However, those backgrounds should not determine the interpretation of a passage if in the process we sacrifice the principle that the Bible is its own interpreter. We should begin with the biblical text itself. Verse 2 describes a legal and permanent union between the sons of God and the daughters of men. Had the reference been to angels, one would have to conclude that they married the daughters of men and became their legal husbands. This is rejected by all serious interpreters.