Seth's line cannot be Sons of God. Seth was a man born of man. Sons of God are born of God. Son's of God are Adam, Jesus, Angels, and we are given the right to become Son's of God through rebirth in the Spirit.
Adam was mud, breathed into by the Holy Spirit to become a living soul. He was directly created by God. Jesus was born of the Holy Spirit hovering over Mary, a virgin, for conception. Seth was a man, conceived of a fallen woman.
We must die to the flesh and be reborn of the Holy Spirit to become Son's of God. Look up the requirements of the High Priest to take a wife. He requires a virgin. The Sons spoken of in Gen 6 are definitely fallen angels. How would the sperm of man create gigantic freaks?
It would help if you looked at all of the reference to Sons of God, meaning those faithful to him.
Seth, Jacob, Joseph, etc.
Try defending your notion that fallen angels mated with humans....
comic book theology!
The righteous line of Seth (sons of God) intermarries with the godless lined of Cain (daughters of men) and become corrupted (except for Noah).
This interpretation appears in rabbinic tradition (Genesis Rabbah 26, 5-7; b. Sanhedrin 108a) and in the Church Fathers ( St. John Chrysostom, Homilies on Geness 22, 8; St. Augustine, City of God 15,23; St. Ephraem, Comentary on Genesis6,30).