Very honest question. That is one of the theories of how the nephilim managed to be on both sides of the flood. There may have been another incursion by the fallen angels or some say they were clinging to the side of the arc. Who really knows. The Scriptures are silent on the issue. The fact is, that there were nephalim all the way to the days of King David. As far as the reasons for the flood, all flesh had been completely "ruined" according to how the Hebrew reads. That includes the animal kingdom as well. So, although rampant violence and sin were part of the issue, the reasons for the destruction went much further. All were supposed to be vegetarians prior to the flood. But the order was perverted. It was so corrupted that He had no alternative but to destroy all living. An act of mercy if you really look at it. He could not allow things to continue as they were. But, Noah found grace and merit in the eyes of God.

I find it hard to believe that the Almighty would have allowed nephilim tainted blood to be on the arc - but then again, the only person the Scripture claimed was of pure DNA was Noah. It was in Noah's merit the others were saved - a perfect picture of our Savior. We are seeing a reemergence of that same corruption that existed before the flood with all this DNA tinkering with animals and humans. "So as in the days of Noah.... So, look up for your redemption draws near." Amen - it is at the very door.