It's not easy to answer your question, because, as you say, the original question is strange. I don't think it is possible to do what you want to do - So, I don't want to offend that person by direct pointing to, that's why I would like to answer more diplomatically but at the same time to drop a hint in a kind and an ironic manner.
Personally, I would probably say something like, "I've never thought about it. I don't see how you can 'prefer' one part of a body".
Of course some young people discuss which parts of the body of a member of the opposite sex attract their attention first (e.g. hair, face, chest/breasts, buttocks, legs), but that's a different situation.
Interested in Language