In brief, it's my understanding that the "sons of heaven," or the "sons of God," refer to the sons of Seth—that is, the descendants of Seth through Enos, Cainan, Mahalaleel, Jared, Enoch of Jared—not to be confused with Enoch of Cain—Methuselah, Lamech, and Noah, the father of Shem, Ham, and Japeth, and the "daughters of man" refer to the daughters of Cain. After the sons of Seth had intercourse with the daughters of Cain, they bore them sons: the Nephilim. It was then that God—to keep the branch of the children of God from being entirely corrupted by the branch of the children of men—sent the universal deluge to extinguish them. It's also my understanding that the discovered skeletons of "ape-men" in a cave back in 1946 were that of the Nephilim.