I have always heard that demons or Nephilim (whichever) aren't actually traveling around in physical aircraft, just that they can create the illusion of a physical object. It's not real, that's why it's so hard to get a picture or any actual proof of a UFO that is convincing. I've also read that abductions are all dreams that demons can project into your mind. I don't believe there are real pregnancies or hybrids walking the earth or anything. But they would probably visit women regularly and make them think they're pregnant or that they had a baby stolen. I don't really believe any of it, but it is a neat thing to think about. It would make a great story. As for the Bible, I can't recall a place that actually states anything to support this concretely.