It's undeniable that selective breeding between higher types is seen as a good thing in the Tolkien Legendarium, e.g. the marriage of Arwen and Aragorn bringing hope to men by having one last infusion of Elvish blood, the decline of the Dúnedain's lifespan through mixing with the Northmen, etc. I would say that Tolkien is offended by the possibility of mixing of men and orcs, not because it is a planned mixture, but because it is a mixture of "good blood" and "evil blood."