Q. Were the ‘sons of God’ (Nephilim) in Gen 6:1-4 fallen angels, and were their children from ‘the daughters of men’ demons?
A. While some would hold to these thoughts, there is no foundation in Scripture to validate the idea that fallen angels produced offspring with human women. Wild guesswork! Your Concordance will tell you that ‘nephilim’, the word used here, means ‘giant’, more precisely a huge man capable of being brutish and bullying. Goliath is a good example; see also the Anakim in Num 13:33.
God created Adam; Eve was made from a rib of Adam. In that context it is acceptable for men to be known as ‘the sons of God’, serving God’s purposes, while women were referred to as the ‘daughters of men,’ supporting men in God’s will.
It may also be considered that the children of Cain, who rebelled against God, would not be seen as the son or daughters of God, as Seth’s progeny was; hence ‘the sons of God’ is correctly applied to Seth’s offspring, while Cain’s offspring are merely ‘the daughters of men’.
In Matt 22:30. Mk 12:25 and Lk 20:35, the New Testament three times records the Lord Jesus saying that angels ‘neither marry, nor are given in marriage.’