1 And it came to pass when the children of men began to multiply on the face of the earth and daughters were born unto them, that the angels of God saw them on a certain year of this jubilee, that they were beautiful to look upon; and they took themselves wives of all whom they chose, and they bare unto them sons and they were giants. 2 And lawlessness increased on the earth and all flesh corrupted its way, alike men and cattle and beasts and birds and everything that walks on the earth -all of them corrupted their ways and their orders, and they began to devour each other, and lawlessness increased on the earth and every imagination of the thoughts of all men (was) thus evil continually. 3 And God looked upon the earth, and behold it was corrupt, and all flesh had corrupted its orders, and all that were upon the earth had wrought all manner of evil before His eyes.

Similarly, Isaac told Jacob: Arise! Go to Padan-aram, to seek a wife proper for him. He did not wish him to be like the men of the generation of the flood, who wallowed in unchastity and indulged in sexual intercourse with species other than their own. Observe what is written concerning them: The sons of God saw the daughters of men … and took them wives, whomsoever they chose. That was the reason they were obliterated from the earth. The animals, likewise, copulated with species other than their own: the horse with the ass, the ass with the horse, and the serpent with the bird, as it is said: And God saw the earth, and behold, it was corrupt; for all flesh had corrupted their way upon the earth. Notice that Scripture does not say in this verse “all man” but all flesh. Therefore, He blotted out every living substance which was upon the face of the ground, both man and cattle. No being that had copulated with a species other than its own entered the ark, as it is said: From all the pure cattle. Because the ark could harbor only pure beings, they came from among those alone, as it is said: Two and two unto Noah.