You can't possibly be offended by being called a Muggle if you are one, because Muggles don't know they're Muggles. You can only be offended if you're
not a Muggle, but by your own admission you don't know what a Muggle is. Ergo, you must be a Muggle.
(Gestapo, by contrast, is a term everybody understands. It is nearly always intended as as insult. Only by appending a

can one neutralize its normally offensive connotations.)