You could maybe try a different approach and ask him "What if I were incontinent and genuinely needed to wear diapers, would you still love me then ? ".
I am pretty sure he would say that of course he would. This may well give you the leverage to then ask him technically what difference would it make if you were wearing diapers for actually need or just because you enjoy wearing them. You are still the same person and diapers are merely a choice of underwear.