People love sensational stories and sadly sometimes look for them in all the wrong places. Genesis 6 is one of those places where from a brief text, some people have come up with all sorts of fanciful stories about angels, demons, and their supposed offspring. But on careful study, these stories are more suited to the Sci-fi channel than careful Bible study.
Here are some useful commentaries on the passage:
The following excerpt is from: http://www.equip.org/bible_answers/did-demons-have-sexual-relations-with-women-in-genesis-64/
Genesis 6:4 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.
Genesis 6:4 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 be not only flawed but also misleading. In fact, it might be logically argued that the disciples did not see the postresurrection 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.”