I think the plain reading of the bible is that these "sons of God" were angels. Anything else is trying to re-explain it because we don't like (or believe) the implications. Jude clearly relates the sin of Sodom (abominable relations) to the sin of these angels (union with women - different kind), so I don't know how the bible could be any clearer on the matter.
Jude 1:6-7
And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day. Even as Sodom and Gomorrha, and the cities about them in like manner, giving themselves over to fornication, and going after strange flesh, are set forth for an example, suffering the vengeance of eternal fire.