Sons of God, daughters of men

• Genesis 6:2 (MSG)

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I've been taught (told) that "sons of God" is in reference to "Angels" or "fallen Angels."
It's an interesting idea (see Wim Wenders "Wings of Desire," "Faraway, So Close!" and "City of Angels" with Nicholas Cage), but where does this idea, theology actually come from?
Couldn't "sons of God" simply be referring to those who followed after God and "daughters of men" simply be referring to those who didn't follow God - perhaps the decedents of Cain?
If "sons of God" were simply fallen Angels, why did God say in the very next verse (6:3) that he would not breathe life into men and women endlessly? Why would he point out "men" versus "sons of God?" 

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