This says that there were mighty men --MEN--thy were men of authority, probably religious leaders--priests, who married women who were not followers of God. Same old story of how have the mighty fallen---still going on, men have a tendency to choose the pretty little vixen instead of the perhaps plain but nice, chaste, women of God.
The word Nephilim, as close as anyone can get, simply means fallen.
There were giants in those days--yep---they lived to over 900 years, nothing says how tall Adam and Eve were. Why do people think they were as tall as we are today?
Angels can not procreate. Jesus made that clear. For a human female to give birth to the offspring of something like what you seem to think these things were---she would have died before she could even give birth. Even a little person carrying a normal sized baby has to have a C section and it is considered a high risk pregnancy. Doesn't anyone think about the implications of such a thing happening? And why in the world would a fallen angel "marry" a human woman? If he could procreate, he cares nothing about the law of God, much less the laws of men--marriage is a religious ceremony. All angels mentioned have male names---God does not approve of same sex unions. So why would God make angels with unneeded "equipment" for copulating with females and not make them any females? You think God has billions of male angels up there with all the proper equipment lusting after human females? That would be cruel. Seriously, it makes no sense. I've even had people say that "Well, it's only the fallen angels that can do this."---Really? You think they can create for themselves body parts that God never gave them? They can not actually create anything. They can only appear to be human or animals, not become one. Besides which, they are of a different order than we. We were created lower than the angels, They are not our kind. Only God could give the that capability and why would He recreate a fallen angel to give them that ability and then turn around and kill them for using that ability?? Doesn't add up any way you slice it.