There are words Giles doesn't use: sweetheart, dearest, darling, beloved. Victorian words, as fussy as lace, as ridiculous as bustles, petticoats, starched detachable collars. Usually he doesn't even think them, in case they slip out.
Not that Oz would laugh. But the words don't fit him any more than a tailcoat would. And Giles can't say baby, honey, something modern that might fit, any more than he can wear trainers.
He says Oz, and Oz says Giles, and there are no other words for this impossibility, this miracle. No words but the naked ones, bare as bodies, bare as love.