Who Were the Sons of God in Genesis?

Genesis 6 talks about the sons of God marrying the daughters of man, the result = giants on the earth. Here are the best options:
1 - Sons of God = the line of Seth (Adam and Eve's 3rd born)
Daughters of man = the line of Cain
This troubles me because it is forced. It does not make good sense. Adam and Eve were human and the results of all their offspring would be normal human beings. To say that the line of Cain mixing with the line of Seth creates giants is impossible to us in the world as we know it, unless the word giant is redefined to mean tall people.

Indeed there are groups of people who are taller than other groups. As one of my students left Viet Nam in a boat, the rumors on the Vietnamese boats spread that the U.S. was a land of giants. Some people returned for fear of the giants. Point is - generally we are a taller people and rumors and I mean rumors make us into giants.

Problem is: The giants of Genesis seem much bigger and making them the sons of Seth and daughters of Cain sounds forced by people who just don't know what else to do with the passage.

2 - The second explanation goes like this: the Sons of God are angels and the daughters of men are humans. This comes from a few scrolls found in Jesus' time that never made the bible. Jesus' brother Jude quotes a lenghty piece from the book of Enoch creating a stir among sholars who question weather or not Jude believed he was quoting from God's word or weather he was quoting a regular book that he did not consider God's word.

The sons of God in Enoch and in another book (Jubilees) are special angels called watchers who were responsible to take care of certain divisions of the world. These watchers lusted after the human women and longed to have sex with them, so they came down to the earth and mated with women. The end result were giants who created havoc for the world of normal humans.

The watchers also did other evils on the earth. They taught men the arts of war and the taught women the arts of make up - how horrid. The results as I mentioned were giants that just about destroyed the world.

Problem with this: Angels only have sexual contact with humans in the world of mythology.