Genesis 6:1-4 (KJV)
1And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
2That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
3And the LORD said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
4There were giants in the earth in those days; and also after that, when 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.
Other sources of text to explore
Book of Enoch, chapter VI, here is a link to this apocryphal book http://www.sacred-texts.com/bib/boe/boe004.htm
Look for yourself in the Bible at various other oints where giants are mentioned and most times in a menacing or threatening way. Joshua and Daniel for instance.
This is not going to be a typical reference about Nephilim; of this book says or that book says. this will be my own interpretation from reading those books. So here we go.
Sexuallity of angels
Another hub I wrote, http://noturningback.hubpages.com/hub/Angels-who-are-they I did quite a bit of research on angels. In this search, I found that they were not created male and female, they were always mentioned as male (Bible) and with no regard as to needing to reproduce.
Giants? I believe these Nephilim/Rephaites/Amalekites were exactly that, at least in the eyes of humans smaller in stature. With seemingly large numbers in population, they disturbed the psyche of all, but two Israelite spies, Joshua and Caleb, who were sent forth to Jericho in Canaan.
If you go back once again to the book of Genesis, we can recall the story of Ham, one of Noah's three sons. This is the son whose offspring, Canaan, was cursed by Noah. Ham's father, Noah, cursed Ham's descendant for Ham having discovered his nakedness and doing nothing about it. Of course being caught with your pants down, so to speak, must have been very embarrassing for Noah and most likely brought about Ham's son Canaan being accursed. You will see that the descendants of Ham, happened to be those who settled in the land of Canaan. This land and their isolation, from the other parts of family through Noah, had likely brought about differences of appearance not seen in the lineage before then. Through adaptations and/or mutations due to their environment, culture and influences from other peoples, quite likely they had changed from the others descended from Noah's two other sons, Shem and Japheth.
The original Nephilim, having corrupted man, must have had the gene carried on through Noah and his offspring. Even though they were wiped out by the flood, they arose once more through this hereditary trait.
Og, king of Bashan
Og, was a king of kings at least as far as stature went, but what would become his downfall, was to rely on his own strength. Most definitely he desired all to be subject under him and when encountering the Israelites, he had chosen to resist their advances and therefore had fallen to Joshua and the Israelites.
Goliath and Lahmi
These two infamous brothers, personify perhaps the latter day Nephilim. I have found mentioned that Orpah, one of the two daughter-in-laws of Ruth, the other being Naomi, had been the birth mother of these two and perhaps a third giant. The forty paces that Orpah had walked with Ruth were a ruse, she had no real intent to stay with her mother-in-law.
Orpah's shame was to lie with many men upon returning to Moab and from this act, she begot these two and perhaps three giants referred to as Philistines.
Goliath, as the Bible reads was 6 cubits and a span tall. Now I have read that to be interpreted to be from nine and a half feet to over twelve feet tall, who's right, does that matter? It is not that critical, what we do know is he was outrageously large as compared to others in his surroundings.