I think, like all things in Tolkien, it depends on a matter of will. That is, possession of such. So we don't see clothes/food etc because Bilbo unthinkingly consider them as *his*; furthermore, inanimate However, we do see Sting, because at that point Bilbo's still very much aware of its elven heritage (also possibly lingering elvish awareness? Tolkien swords can be a bit sentient?). However, someone ELSE'S anatomy is necessarily someone else, so it would be like Boromir reaching for Frodo- his leg doesn't disappear Frodo kicks him etc. However, if maybe the POSSESSION seems obvious and matter in fact(required for certain dynamics perhaps), only then would I argue for disappearing cock.