"The Nephilim were on the earth in those days — and also afterward — when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown." Genesis 6:4
Genesis 6:4 is one of the most controversial passages in the Bible. As with any difficult section of Scripture, it has been open to a wide variety of interpretations. It is my conviction, however, that those who hold consistently to a Biblical worldview must reject the notion that women and demons can engage in sexual relations. I reject this interjection of pagan superstition into the Scriptures for the following reasons.
First and foremost, the notion that demons can "produce" real bodies and have real sex with real women would invalidate Jesus' argument for the authenticity of His resurrection. Jesus assured His disciples that "a spirit does not have flesh and bones, as you see I have" (Luke 24:39, NKJV). If indeed a demon could produce flesh and bones, Jesus' argument would not only be flawed, it would be misleading. In fact, it might be logically argued that the disciples did not see the post-resurrection appearances of Christ but rather a demon masquerading as the resurrected Christ.
Furthermore, demons are nonsexual, nonphysical beings and, as such, are incapable of having sexual relations and producing physical offspring. To say that demons can create bodies with DNA and fertile sperm is to say that demons have creative power — which is an exclusively divine prerogative. If demons could have sex with women in ancient times, we would have no assurance they could not do so in modern times. Nor would we have any guarantee that the people we encounter every day are fully human. While a Biblical worldview does allow for fallen angels to possess unsaved human beings, it does not support the notion that a demon-possessed person can produce offspring that are part-demon, part-human. Genesis 1 makes it clear that all of God's living creations are designed to reproduce "according to their own kinds."
Finally, the mutant theory creates serious questions pertaining to the spiritual accountability of hypothetical demon-humans and their relation to humanity's redemption. Angels rebelled individually, are judged individually, and are offered no plan of redemption in Scripture. On the other hand, humans fell corporately in Adam, are judged corporately in Adam, and are redeemed corporately through Jesus Christ. We have no Biblical way of determining what category the demon-humans would fit into — whether they would be judged as angels or as men, or more significantly, whether they might even be among those for whom Christ died. I believe the better interpretation is that "sons of God" simply refers to the godly descendants of Seth, and "daughters of men" to the ungodly descendants of Cain. Their cohabitation caused humanity to fall into such utter depravity that God said, " 'I will wipe mankind, whom I have created, from the face of the earth — men and animals, and creatures that move along the ground, and birds of the air — for I am grieved that I have made them.' But Noah found favor in the eyes of the Lord." (Genesis 6:7-8).
For further study, see Hank Hanegraaff, "Questions and Answers: Genesis 6:4," available from CRI at www.equip.org; see also Gleason L. Archer, New International Encyclopedia of Bible Difficulties (Grand Rapids: Zondervan, 1982), 79–80.
Adapted from Hank Hanegraaff, The Bible Answer Book (Nashville: J. Countryman, 2004).