The "fallen ones" sought to merge with the bloodline of Adam, because of the promise to send a redeemer through Adam's kinsman. The Hebrew says that the Sons of God saw that the women were a fit "extension", for they sought to extend themselves into this realm from the spirit realm, as well as to extend themselves into the "children of the promise" the lineage of Adam. Satan tried to prevent the eventual birth, in the distant future, of the Messiah.
The Bible uses other names to describe these degraded fallen angels and their descendents in addition to the word Nephilim, they are. . . .
Rephaim - from the root rapha = spirits, shades Gen. 14:5
Anakim - Race of Giants Num. 13:33
Descendents of Nephilim Emim - the proud deserters, terrors, race of giants Gen. 14:5
Zuzim - the evil ones, roaming things Gen. 14:5
Zazummims - the evil plotters, Deut. 2:20
Zophim - watchers, angels who descended Num. 23, distinct from 'Holy Watchers' aligned with God
Sepherim - the many. . . .
Give me a link to the web site. I used to have about 10, but deleted them. I wonder if it is one of the ones I used to have bookmarked? BTW, wasn't Goliath one of these descendants? I think the Bible says they had six fingers and six toes, and their spears were like a weavers beam.