Bitcoins are not interchangeable if a recipient decides not to accept them, whether because of an identifiable history or some other reason.  Monero is not interchangeable if a recipient decides not to accept them, whether because of a lack of identifiable history or some other reason.  It all boils down to which currency will be the most widely accepted, but I will concede that monero is technically more fungible. 

As far as gold being an example, American Eagle gold coins are considered legal tender in the United States and guaranteed by the US government.  As a result, they sell for a slight premium to other gold coins, effectively making gold not completely fungible.