hmmm... something about the Sons of God mating with the Daughters of Man?
Perhaps. I was thinking more along the lines of "anything that moves."
my thoughts immediately went there too...
The sons of God - Those who were called by the name of the Lord, and called upon that name, married the daughters of men - Those that were profane, and strangers to God. The posterity of Seth did not keep to themselves as they ought, but intermingled with the race of Cain: they took them wives of all that they chose - They chose only by the eye: They saw that they were fair - Which was all they looked at.
The Sons of God were not angelic beings. (It looks like you are suggesting this. I know others have.) Gods makes it clear in Genesis that the kinds reproduce after their own kind. (Genesis 1:21,24,25). Men and women reproduce within their own kind. Angels do not marry and do not reproduce.
"Sons of God" are men who believe on The Lord (see John 1:12). The "daughters of men" are the unsaved females of the unbelieving tribes/groups. God warns us not to be unequally yoked. He told the children of Israel not to interbreed with the occupants of the promised land. And he repeats this in the NT when he commands us not to be "unequally yoked."