I read the passage and read it in context to help with interpretation. The sons of God here is referring to the same fellows in Gen 6:2 who are listed CONTRASTING the daughters of men. These beings were indeed NOT of this world as the writer would have stated man, as it does in vs 5, 6, 7, 9, etc...
There is enough passages to refer to the fact that indeed heavenly beings created by God do choose between right and wrong and interact in the world of men.
Are they demons...I wouldn't go as far to say that...but they very well could be...and these giants are but offspring of these unions between them and the daughters of men. There is no mention here of female "celestials" if you will. It seems specific enough to take that into account here.