If you simply assume that God continued to create new people beyond Adam and Eve, this makes a lot of sense. The "sons of God" were those not born but created. That daughters being born is noteworthy suggests that women were being made some other way. This also solves the whole problem of how Cain could know his wife without her being his sister.
Question; If one of the curses to the woman in Genesis Chapter 3:16 was a increased sorrow in bringing forth children, were children born prior to the fall? and why did Eve (who was named Eve after that carastrophe) say she had gotten a man from the LORD when she bore Cain, could this mean daughters were born and not recorded (it seems the Bible records male geneaology for the record in the OT)? This of course does preclude that daughters were born prior to Cain's problems but after the fall.
You do raise an interesting issue however.