You need citations, please. Angels have animalistic characteristics? No more than humans. Can humans create offspring from sex with any known animal? If God were so much against it then He could've simply followed the same rule as applied to every other animal species.
Ezekiel 10:14 Each of the cherubim had four faces: One face was that of a cherub, the second
the face of a human being, the third the
face of a lion, and the fourth the
face of an eagle.
I'm not trying to argue with you. Just trying to give a different perspective, so I hope you don't take it the wrong way. If the above is intended to be taken literally as a physical description, then I'd say they're DNA was somewhat similar to a human's and somewhat similar to an animal's. Although there are drastic differences. haha
As for destroying all life on Earth... God gave everyone the ability to obey him or to sin. Those who sin are condemned to destruction unless they accept Christ. But the account in Genesis does not state intermingling as the primary reason for the destruction of those human beings.
Genesis 6:5 The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only
evil all the time.
One could speculate that the intermingling had something to do with the destruction, but I don't think it was the primary reason. Also, you might be interested to find that the liger is not really the exception to the rule. I hesitate to comment further, since my memory is a bit hazy on this point. But I think if you looked you could find similar situations with other creatures - even plants.