I agree that it's only the relation that's problematic, but that's the thing. Only the mind can determine what the relation ought to be, because only a mind can have a concept of "ought". The body can't disagree, it can only fail to meet expectations; it's an object (an exceedingly important one, mind). I can't think of a justification for insisting that the mind should conflate what is with what ought be.
By this logic, if I have a delusion that my body must have wings, the problem is with my body, not in my head.
Well, yes, actually, I would agree with that, and would fully support you in your quest to obtain wings, however difficult that might be. Especially if the lack thereof was causing you severe mental distress and didn't seem to be interfering with your perception of reality. If you believe you
do have wings, that's a different problem, but if you believe you
ought to... well, who am I to say otherwise.
Your body has told you how it is, but you're free to define what ought be in whatever way seems best to you. I might argue over what is or is not a practical course of action, but I'm certainly not going to insist that you're wrong for wanting this. You wish nobody harm, and as far as the situation so far is concerned, are harming nobody in pursuit of your goal.
Ninja'd, apparently; gotta go for a bit but I'll read em when I get back.
EDIT: Couple of things I need to clarify, I suppose. First, "free to define" comes off a bit different from the intended meaning, but the idea of pairing that with "in whatever way seems best" was that while you might not have absolute control over what seems right to you, you are under no obligation to conform to another's ideas of what ought or ought not be the case.
Second, I should point out that while I do get where you're coming from with the arbitrary nature of it, and you have a good point, whether it's classified as a mental or physical disorder (or whatever else) tends to determine how treatments are designed. Possibly I'm a bit odd, but I prefer to change a body rather than a mind if it's at all possible.