As for Tolkien, what on Erda did you all think happened in Saruman's breeding pits? I was utterly gobsmacked when I saw Peter Jackson's "orc cyst" thingummy. Tolkien was too discrete to detail most of the atrocities committed by orcs, but I never doubted what that was indicating forced orc-human interbreeding.
Some of the last writings by the Professor on the origin of the Orcs and their kind go into some of the difficulties and horrors associated with the project:
[quote="Morgoth's Ring", J.R.R. Tolkien, p. 418-419]
It became clear in time that undoubted Men could under the domination of Morgoth or his agents could be reduced almost to the Orc-level of mind and habits; and then they would or could be made to mate with Orcs, producing new breeds, often larger and more cunning. There is no doubt that long aftewards, in the Third Age, Saruman rediscovered this, or learned of it in lore, and in his lust for mastery committed this, his wickedest deed: the interbreeding of Orcs and Men, producing both Men-orcs large and cunning, and Orc-men treacherous and vile.[/quote]
The problem, IMO, is that D&D has orcs without the background Tolkien gave them--Tolkien's Orcs are monsters in the literal sense, Elves (or possibly Men or hominid beasts) warped in body and mind by the dark arts of Morgoth, Sauron, Saruman, and others who have rebelled against Eru and His Music. D&D orcs are typically on the same order of being and nature as any other sapient race, and thus not necessarily driven towards evil in the same way that Morgoth warped the Orkor.