While it clearly states that the "the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown" this is also echoed by the Pharoahs of Egypt, who called themselves "sons of God" to enhance the power they had over their subjects, calling themselves deities. However, this is a point made by an overtly Christian resource
. When recalling this, that the Pharoahs called themselves sons of God, and that the Bible clearly references the "sons of God [that] came in unto the daughters of men
" it's difficult to pass off the notion that the nephilim were extraterrestrial. Indeed, the chapter in question notes that there "were giants [nephilim] in the earth in those days; and also after that, when the sons of God came in unto the daughters of men..." decidedly inferencing the idea that the nephilim were there BEFORE man, and came to them after the fact.
The idea of extraterrestrial origin and intellectual design is hot button, because of multiple theories which state that it's impossible, and also of clearly religious import and design. One theory is that saying we were created by a creator leads to the necessity of that creator being CREATED by another creator, and on and on and on, which therefore makes proving it impossible and highly unlikely. And multiple court rulings have stated that intelligent design (by deity or alien) is a matter of religion, and as religious ideas have been ruled to not be taught in science classrooms, so to does that idea go out the window.
People, I think, will discuss it, but should you bring it up in any "serious" scientific conversation, it'll be laughed at and dismissed.