Fungibility is an important property of any functioning currency. You can try to hide your bitcoins as much as you want, if you tried to mix your non-fungible coins using a mixer, coinjoin or another type of "anonymity enhancing feature", these transactions can still be flagged as "possible suspicious activity on the blockchain", even if you are anonymous. Using non-fungible tokens as currency can eventually lead to blacklisting/whitelisting either by governments or through self-censorship. Some examples of these measures could be payment processors or exchanges refusing your tainted coins as a payment or deposit or miners refusing to include your suspicious transaction. Monero will enforce a minimum mixing across the network, so all outputs are mixed by default. This is possible due to the nature of the mixing: monero mixing is "passive" and can even be done offline! Transaction outputs have "plausible deniability" about their state: you can't tell if they are spent or unspent in a certain transaction or not. This leads to an opaque (non-transparent) blockchain making all coins "equal". Fungibility is built into Monero at protocol level, making it real "digital cash"..