Ask yourself ,who were they marrying?
Let me answer my own question,the angels
Genesis 6:1 "And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,"
Genesis 6:2 "That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose."
The "sons of God" are angelic beings, or angels, and their offspring are called "the Nephilim". These angelic beings, or angels saw the daughters of men were beautiful. These are the fallen angels of Jude 6, who followed Satan in his fall in the "world that was", which is the first earth age. Jude 6 reads; "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."
We see that it is these fallen angels that are already reserved undo judgment, and continuing to follow Satan's, in their coming to earth, and intermixing or marring Adamic daughters. This may be strange to you, However, it is taught by Paul in the New Testament. Paul was warning Woman in
I Corinthians 11:10; "For this cause ought the woman have power [a sign of authority] on her head, because of the angels."
The protection, or power of authority Paul warned against was from the angels. Though many translate this covering to be "hair", however, it should be veil. That veil is the "Holy Spirit". Because in the latter days it is going to be just as it was in the days of Noah, and friend we are in the latter days. If both Jesus and Paul warned the Christians to be alert of this fact, we had better be alert of it.