That's a huge leap of logic to assume they were not fully men and therefore unredeemable when the scriptures in many places calls them men.
Ok so I have read a lot of these replies and there are a few thing I might be able to contribute to help folks understand about the "fallen angels".
First of all there is no redemption for any of the fallen angels because they knew what the were doing and what the were giving up when they Grace. When they gave up their habitation and former estate they went from immortal to mortal and were no longer allowed back into heaven, "The abode of God". Can you imagine how surprised they must have been when they discovered that they were changed from immortal beings into mortal giant men who could be killed because of their bad choices.
Jud 1:6 And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
The fallen angels willingly gave up their "Former Estate" which was immortality and an eternity of praising God and they willingly left their "Own Habitation". They gave up living with God in heaven to live here on earth. Angels were created to live for ever.
Job 38:6 Whereupon are the foundations thereof fastened? or who laid the corner stone thereof;
Job 38:7 When the morning stars sang together, and all the sons of God shouted for joy?
These two verses from the book of Job are talking about a time before the fall of Satan and his angels. In verse 38:7 when it says "When the morning stars sang together" it is talking about both Satan and Christ singing together in praise to God and when the verse says "all of the sons of God shouted for joy" it is talking about when "ALL" of the angels shouted for joy as one third of the angels fell from Grace to follow after Satan.
But the angels in Tartarus are yet another group of fallen angels. I know that the book of Enoch is not in the Bible but it does exist. In fact there are 6 books mentioned in the Bible that are not in the Word of God. Anyways in the book of Enoch it talks about how God sent 200 angels down to earth to help mankind, however the angels taught mankind the "Ways of the plants" and what did we get from plants??? Drugs. So it is my belief that the angels reserved unto judgment will also be judged at the Great White Throne Judgment.
2Pe 2:4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
So there are three groups of Angels, The heavenly host, The angels who were cast down to earth and the Angels reserved in chains unto judgment.