He's right about the language issues of the Genesis passage. In the entire Old Testament, the Hebrew for "Sons of God" used there is always used for the angels. It makes for a somewhat uncomfortable interpretation.
The sons of Cain / daughters of Seth idea came later on. It does seem to have issues. Not just with the use of the words themselves, but with the idea of the Nephilim, and the use of the words "giants" just a bit later on. That word is really a transliteration of the Greek "gigantes," which means "the Earth born."
It puts an interesting spin on Jude 6, the angels which "kept not their first estate." The Greek used here means dwelling place, and can refer to a physical body.
Overall, it is an interesting theory, and I think it is possible. I do not say that it must be true. I think that we may find errors in the theory, but there just may be something to this.
1And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
2That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
3And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
4There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
5And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.