ANSWER: Your question concerns Bible verses that are well known to many who study God's word. The Nephilim did indeed exist before Noah's flood. These giants came into the world when the earth was experiencing a population explosion (Genesis 6:1). Just how many people lived just before the deluge? The period from Adam's creation (about 3984 B.C.) to just before the waters covered the globe (2328 B.C.) is roughly 1,650 years. Conservative estimates place the population of the world around one billion!
4 In those days, and even later, there were giants (Nephilim) on the earth . . . They were the great heroes and famous men of long ago. (Genesis 6:4)
One common question is who were the "sons of God" who married women and produced these giants? Some believe they were demons, fallen angels who took the form of human flesh so that they could produce giants. This theory does not make any sense, since there is no place in the Bible where God calls demons his "sons."
There is another problem with the idea that angels (either righteous or fallen) and women were able to produce a race of giants called Nephilim. The Bible says the sons of God took WIVES for themselves, meaning they were MARRIED, and together they produced giant children (read Genesis 6:2 again). It does NOT say these women were raped or otherwise violated sexually before marriage and had children out of wedlock.
Marriage is an institution God began in Eden (Genesis 2:24 - 25) that receives its authority and binding nature DIRECTLY from him in heaven (see Matthew 19:1 - 6). There is not even a HINT in scripture that God ever spoke about, conducted, or approved of angelic beings somehow 'marrying' humans. Those who argue that these 'sons' were angels have yet to produce proof that God has expanded his will regarding marriage, and the producing of potential spiritual sons and daughters, to angels.
Additionally, Jesus tells us something very interesting about angels that relates to the Nephilim. One day the self-righteous Sadducees came to Jesus with a question they hoped would entrap him and justify an arrest. They asked that if a woman had seven husbands, and no children from any of them, whose wife would she be at the resurrection of the dead. Jesus, knowing their hypocrisy (Sadducees did not believe in a resurrection) answered them with the following.
30 For when the dead rise to life, they will be like the angels in heaven and will not marry (Matthew 22:30)
Jesus tells us clearly that those who attain the resurrection of life will not marry (or be capable of having children), but will be like the angels. Angelic beings are spirit and unable to reproduce. Only human beings in the flesh have the gift of marriage and procreation.
The Bible informs us, in places other than Genesis, who God considers sons (John 1:12, Romans 8:14, 1John 3:2). Only those who exist with the indwelling of the Holy Spirit are truly God's sons. The sons who married the "daughters of men" that ultimately produced the giants were righteous men who foolishly married worldly women. Note again that verse 2 of Genesis 6 says, "they took wives for themselves whomever they chose." Instead of relying on God to guide their marital decisions, righteous men before the flood decided to pick mates THEY lusted after. They became, as the apostle Paul states, "unequally yoked" with unbelieving women (1Corinthians 7).
In conclusion, giants known as the Nephilim did exist before the flood.