When man began to multiply on the face of the land and daughters were born to them, the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose. Then the Lord said, “My Spirit shall not abide in man forever, for he is flesh: his days shall be 120 years.” The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of man and they bore children to them. These were the mighty men who were of old, the men of renown. - Genesis 6:1-4
Unlike the ambiguous "sons of God," I'm pretty sure this refers to normal antediluvian human women. The sons of God found them attractive and took wives from among them. When they went into them, they bore children to them who were mighty men. The Nephilim were on the earth at this time, but I don't think the text necessarily says they were their offspring.
Why is a solution to this scheme (besides the Flood) to limit man's years?