So, if God and Mary had a son and he was the son of man and it was all good, then why was it a problem when the sons of God and children with human women????
The sons of God were those who believed God in those days, as today. Another way to say it is
children of God. God the Father is your father when you believe him, through Christ. In the days of Genesis, there were sons of God, too, i.e., people who believed God. What makes Jesus different in this respect is that he's God's only
begotten son. The
daughters of men were those who were not God's children, i.e., those who didn't care about Godliness.