In view is the “in those days, and also afterwards” portion of the Bible’s Genesis ch. 6 text. Of course, delving into the minutia of this text would take us far and beyond the confines of one article thus, let us quote the relevant portion and focus on the incursion issue.
This is Genesis 6:1-5 from the NASB:
Now it came about, when men began to multiply on the face of the land, and daughters were born to them, that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose.