When human beings began to increase in number on the earth and daughters were born to them, the sons of God saw that the daughters of humans were beautiful, and they married any of them they chose.
Genesis 6:1-2 (NIV)
There are 2 schools of thought on the “sons of God” in Genesis 6. This phrase “benei ha-elohim” can either be:
1. Angels who impregnated women
2. Men of Seth’s “godly” lineage who mixed w/ Cain’s line.
In interpretation 1, the Nephilim are monsters and giants. In interpretation 2, they are evil humans.
Either way, whether you believe the “sons of God” refers to angels or men in Seth’s line, neither position requires you to believe that the events actually happened.
The goal is to try to understand what the text intends to teach us, and the lesson may be the same either way.
That said, I strongly believe that Genesis 6 is meant to tell us a story about angels who took human form and impregnated women. Not that I think it literally happened, but I think that’s what the story is saying. And this is primarily because “daughters of Adam” cannot just mean “daughters of Cain.”
Besides, Jude and Peter aren’t going to quote from the Book of Enoch if they don’t think their audience is familiar with the book of Enoch and understand what says. And what it says is that angels impregnated women and those women gave birth to monsters. Later, war. And then the flood.