Well, it's not really clear who's doing the sexing. It's from Genesis, chapter 6: "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."

It's a bizarre passage that is never really explained in the Christian Bible. It's probably fleshed out in some Torah commentary, but I don't know much about that.