There are two main views on Genesis 6.
One that says that the sons of God are referring to the line of Seth who went with the woman of Cain. basically marrying those who had rejected God.
The other and possibly Older, and certainly held by the early church was that the sons of God here refer to fallen angels (Using Job 1:6 etc as proof that angels where called sons of God).
Which view do I hold well I'm not saying