it's talking about the angels. there were some angels who came down and had intercourse with the women and made giants. read jude v6, 2 peter 2:4. These are not the angels that fell with satan. angels are sons of God. 

Here in Job 1:6 we also see a reference to the sons of God. We see here that satan came among them. Now we know that satan is a spiritual being and not a man. From that, I conclude the sons of God were spiritual beings.

Jesus when He walked this earth usually used the term son of man to refer to humans. 

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. 

I have heard it preached that they were angles and that is how giants came into being.
My opinion would be to do this... Try to find out what the original Hebrew words were, along with their definition, for the phrase "sons of God" in that verse and then check to see what the original Greek words were, and their definition, in the verse in 1 John: "Behold what manner of love the Father has given to us, that we should be called the "sons of God". That way you can better make the determination that the Bible meant either Angels or devote followers of God. 

The sons of Godin this verse were humans who claimed to be the obedient spiritual servants of the one true God.
As John said in 1 John: "Behold what manner of love the Father has given to us, that we should be called the "sons of God" "

They were not angels or demons as many claim. These were simply the spiritual professed sons of God who apostatized and decided to commit polygamy.