Video from Got Questions.org
"Who were the Sons of God and Daughters of Men in Genesis 6:1-4? Did the fallen angels mate with human female to produce the Nephilim?" from video introduction
The three primary views on the identity of the Sons of God are 1) they were fallen angels, 2) they were powerful human rulers, or 3) they were godly descendants of Seth intermarrying with wicked descendants of Cain. Giving credence to the first theory is the fact that in the Old Testament the phrase “Sons of God” always refers to angels (Job 1:6; 2:1; 38:7). A potential problem with this is in Matthew 22:30, which indicates that angels do not marry. The Bible gives us no reason to believe that angels have a gender or are able to reproduce. However we must realize the bible says they do not marry it never says thy do not reproduce.
Here is a link to the website True Free Thinker entitled Michael Heiser on “Who (or What) Were the Nephilim?”