The idea that Genesis 6 was refering to the Sons of Seth marrying the daughters of Cain is absurd. This idea was not present until the Second Century A.D. when a man by the name of Julius Africanus became embarassed by the whole idea of angels and people having sex. This false idea then began to be passed along and accepted by the masses. There are a lot of weak points to this argument that totally destroy it. The term sons of God (B'nai Ha Elohim) is only used to refer to angels in the Old Testament. (Job 1:6, 2:1) are two examples. You can search the Old Testament for eternity and never find one reference to men being the sons of God. The Next bit of evidence that they were angels comes from (Gen. 6:4) The phrase daughters of men (benoth Adam) does not differentiate between Cain or Seth. No where in the whole text are the names Cain or Seth even mentioned. Another bit of evidence it that Cain repented and was forgiven by God, He even put a seal of protection on him (Gen. 4:15). Seth on the other hand gave birth to a son named Enos. In the english it says, "then began men to call upon the name of the Lord" (Gen. 4:26). In the Hebrew text it says men began to profane the name of the Lord. That destroys the argument that Seth's line was the "Godly" line. Jude and Peter both associate the angel's sin with that of Sodom and Gomorrah, it was sexual in nature (2 Pet. 2:4)(Jude 6-7). Further evidence is that all the ancient Jewish scholars KNEW this verse referred to angels. They knew thier language better than we do. There is also a ancient Jewish legend about the angels of water and fire coming down to earth and having sex with women, it says this was the reason for the flood. Supreme proof that giants existed is in almost every cultures account of the matter. The Jews, the Greeks, the Germans, the Japanese, etc. We live in an age where people discount ancient beliefs as just myths, but there is more than enough evidence to prove that giants once existed because of these mischievious angels, and one statement that Jesus made proves that if the days of Noah were like that, then it will come to pass in this age. "But as the days of Noe were, so shall also the coming of the Son of man be" (Matt. 24:37). I hope that this shed a little light on the subject, but if it didn't, a good place to start researching here.