Sons of God/ Nephalim

There are many groups that talk about Nephallim as being half spirit and half human as well as incubi and succubi, but as the article points out, created spirit beings do not have the ability to reproduce and therefore the phrase "Sons of God" cannot apply to angels or demons.

http://www.ucg.org/brp/brp.asp?get=daily&day=6&month=February&year=2002&Layout=
Sons of God, Daughters of Men, and Giants (Genesis 6)Some have taught that Genesis 6 describes fallen angels interbreeding with human women to produce half-demon giants. But there is a more rational explanation.Halley's Bible Handbook states that "the 'sons of God' (6:2) are thought to have been either fallen angels or leaders in Sethite families who intermarried with the godless descendants of Cain" (24th ed., p. 72). The first possibility offered here is not really a possibility at all, even though angels are referred to as "sons of God" in Job 38:7 because God is their "Father" through creation. Angels are spirit beings (Hebrews 1:7), not fleshly creatures. They neither marry nor sexually reproduce (compare Luke 20:34-36). Also, this explanation would violate the principle made clear in Genesis 1 that each kind reproduces only "according to its kind." Furthermore, the risen Jesus explained that "fallen angels," or demons, are not able to manifest themselves materially like He and the righteous angels can (Luke 24:39; compare verses 40-43; Genesis 18:1-8, 16; 19:1). Rather, we see demons in Scripture only possessing individuals or appearing as ghostly apparitions.The second explanation in Halley's is far more reasonable and better fits the context of the passage. Genesis 4 gives the story of Cain and Abel and follows with the genealogical descent from Cain. Genesis 5 is called "the book of the genealogy of Adam" (verse 1). It starts with God's creation of Adam and how Adam's line continued through Seth. As with the angels, Adam was a "son of God" by creation (compare Luke 3:38)—though even more so since Adam was made in God's image (Genesis 1:26; 5:1-3). Of this family line through Seth it is stated, "Then men began to call on the name of the Lord"—which could also be rendered "called after the name of the Lord." Then, in the next chapter, Genesis 6, we see "the sons of God" (men of Seth's godly line in this explanation) intermarrying with "the daughters of men" (women of Cain's ungodly line).There is even a third possibility, in which "sons of God" should be translated "sons of the gods," as the Hebrew word elohim here, plural in form, can sometimes refer to false gods instead of the true God. In this explanation, wicked men referred to as sons of the gods (either pagan worshipers or perhaps men claiming to be demigods themselves) forcibly "took" innocent women as wives—an example perhaps of the evil conduct of the day.In any event, human beings were clearly the problem here—not angels. God says, "My Spirit shall not strive with man forever" (verse 3) and "I will destroy man whom I have created from the face of the earth" (verse 7). Thus, the giants mentioned must have been human also—descendants of Adam and Eve (compare Acts 17:26). These very tall people were destroyed in the Flood. But there would be more like them following the Flood, who were descended, just as everyone else in the post-Flood world, from Noah—again, not angels (compare Deuteronomy 2:20-21; 3:11). Consider Goliath, whom David slew. He was more than nine feet tall (1 Samuel 17:4). But he was still just a man (verses 24-25, 33)—not some human-demonic hybrid.