Okay, let me try this way:
Monero is perfectly distinguishable from any other forms of money therefore if some social convention somewhere prescribes they will not accept it then it is non-fungible.
"Fungibility refers only to the equivalence of each unit of a commodity with other units of the same commodity. Fungibility does not relate to the exchange of one commodity for another different commodity"
https://en.wikipedia.org/wiki/FungibilityAlso, from the same source:
"A good is fungible if one unit of the good is substantially equivalent to another unit of the same good of the same quality at the same time and place."
Notice the wiggle word in there. Fungibility is subjective.