Regarding the Nephilim mentioned in Genesis. We can only speculate on this matter. You have to search out these matters from other other ancient writings, sort out, and piece things together. That's the best one can do...
Most likely the Fallen Ones did not have sexual relations with women. Jesus mentions in the gospel account that angelic beings essentially cannot do this as they do not have gender.
Most likely they altered DNA and artificially impregnated women is what is meant. Next Jesus said the last days will be like it was in the days of Noah. Look at the advances in DNA manipulation and what do you see? You see more sexual perversion gloried as normal and healthy, people becoming more violent, etc.
I do not want to sound like the Ancient Alien TV host or show as in my opinion they are off base. However, there maybe a little truth in all the doo doo they talk about.
What we do know from the bible is that there was a rebellion in Heaven, the focus point is earth where the fallen ones (demons) attempt to prove God cannot keep his word, promises, and gifts so their leader can exalt his throne above God's. They do so with humanity as a tool for their ends.
Bible speaks about spiritual warfare. That part is true. For fallen angelic beings to come and attempt to alter DNA of human beings is possible. Their appearing to folks as beings of light and selling the idea is possible. They deceived the folks in Noah's day - so why wouldn't it be any different now since Jesus mentioned that the last days will be like the days of Noah?
Food for thought...
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