The traditional Evangelical interpretation which you will find in commentaries like the Geneva Study Bible, Matthew Henry, Wesley's Notes or JBF, is that it refers to intermarriage between between the godly sons of Seth and the irreligious daughters of Cain.
Another approach reads the 'sons of God' are angelic beings, which raises the obvious question of how angels could have sex with human women and have children. How are two completely different lifeforms even interfertile? Clearly there is one interpretation that says "they just are". It is an interpretation that contradicts what Jesus tells us about angels and what we know about biology.
Another interpretation take the sons of God as rebellious angels but sees them using a human host, a demon possessed man, to have sex with human women. The angelic involvement is spiritual, the biology is totally human.
Then we have the metaphorical meaning. We find throughout the bible the ungodly referred to as sons of the devil, or children of Belial. It does not mean literally. The Pharisees Jesus said were of their father the devil, had normal human mothers and fathers. But since we are talking about Genesis here, don't forget the ancient promise that God would put emnity between the seed of the serpent and the seed of the woman. When the Pharisees acted as children of the devil hating Jesus, they were fulfilling this ancient prophecy. But it is their characters not their biology that are children of a fallen angel.
So if this metaphor of children of a fallen angel is found in Genesis 3, isn't it possible when we read of children of fallen angels in Genesis 6 it is the same metaphor? In fact the passage in Genesis 6 echoes the language of the creation account with God saying I will destroy Adam whom I have created from the face of the earth Gen 6:7.