If daughters were just being born to them, then who were they having all these children with??

"Genesis 6:2 tells us who fathered the daughters...."The sons of God (angels) saw that the daughters of men were fair, and they took wives of all they desired and chose". Jude 1:6 tells us what God did to these angels, "And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day"."

?? Says it right there. Daughters of men.