It certainly seems something strange and outside of God’s reproductive scheme for mankind went on before the flood… and it would seem that it was imperative for God to destroy the world before it became so ‘contaminated’ that there would eventually be nobody alive who didn’t have some genetic trace to the ‘Nephilim’. Clearly God couldn’t allow that to happen since that would eventually also include the mother of the Messiah which God knew would come to be the saviour. However, I don’t think that Matthew 24:37 has anything to do with genetic manipulation (But as the days of Noah were, so shall also the coming of the Son of man be.) I think that this just has to do with the state of the world (violent), the fact that scoffers and non-believers will abound and it also seems to carry a comparison to people hearing truth and knowing what they should be doing spiritually… but rejecting it. Remember that Noah preached that ‘the flood was coming’ for one hundred years before it actually happened… and when it did, it came so quickly that mankind was wiped out. And when Christ returns to claim his own, it will all be over before anyone even knows what happened…in fact, it will be even faster than the rains coming.