And Nephilim happened before Genesis 6.
1Now when men began to multiply on the face of the earth and daughters were born to them,
2the sons of God saw that the daughters of men were beautiful, and they took as wives whomever they chose.
3So the LORD said, “My Spirit will not contend with man forever,
a for he is mortal; his days shall be 120 years.”
4The Nephilim were on the earth in those days—and afterward as well—
when the sons of God had relations with the daughters of men. And they bore them children who became the mighty men of old, men of renown.
The Nephilim were on the earth in those days when the sons of God had relations with the daughter of men. And they bore them ( a pronoun referring back to the subject previously mentioned; that is, the Nephilim) children (who became the might men of old, men of renown.
So, not only were the sons of God taking wives whomever they chose in those days, so were the Nephilim, when men began to multiply on the face of the earth. But, why should the sons of God be equated with the Nephilim as if they are the same group?