"Now it came to pass, when men began to multiply on the face of the earth, and daughters were born to them, that the sons of God saw the daughters of men, that they were beautiful; and they took wives for themselves of all whom they chose. And the LORD said, 'My Spirit shall not strive with man forever, for he is indeed flesh; yet his days shall be one hundred and twenty years.' There were giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them. Those were the mighty men who were of old, men of renown" (Gen. 6:1-4).
As God desired, the human race was growing rapidly and filling the planet. Some have estimated that at the time of the flood, there may have been one billion people alive on Earth (compare that to the 7 billion who live here presently). However, with each passing generation, it seems that righteousness is pursued less and less. The reference in 6:3 to 120 years is not describing a shortened lifespan for mankind. Rather, God is declaring that in 120 years He is going to stop putting up with the wickedness of man. The human race would become extinct (almost); there would be a fresh beginning.
6:2,4 have caused plenty of imaginations to run wild with speculation, particularly the idea that angels mated with human beings at one point in the past ("the sons of God came in to the daughters of men"). There is no denying that there are passages in God's word that refer to angels as "sons of God" (e.g., Job 1:6), but such is not demanded here and goes against Jesus' statement about angels not being sexual beings (cf. Matt. 22:30). Besides, the phrase can refer to mortal men (cf. Hosea 1:10). It makes little sense to affirm that angels noticed how attractive the daughters of men were, so they married them and had intimate relations with them. The most reasonable interpretation is that the phrase "sons of God" simply refers to those of Seth's lineage who followed God. These men worshiped God publicly (cf. Gen. 4:26), walked with Him (cf. 5:24), and looked to God for comfort (cf. 5:29). The "daughters of men" generally refers to those of Cain's lineage who were worldly and corrupt. The problem here is that those who had been pursuing godliness become more concerned with physical beauty than with character! That was (and is) a big mistake. This intermarrying would produce general corruption. Soon there would be very few who were seeking to walk properly with God!
But what about the "giants", Stephen? Doesn't that prove that the angels mated with human women? Of course not! A careful reading of 6:4 shows that the "giants" were on the Earth before"the sons of God came in to the daughters of men" (this can be deduced because of the way the phrase "also afterward" is used). So, they can't be angel offspring. There are some human beings today who are very large and tall. Typically, it is a matter of genetics. Of course, the Hebrew term here is nephilim conveying the idea of attackers, robbers, bandits, tyrants, etc.