Correct about diamonds. Incorrect about cryptocurrncies. Fungibility means that units are interchangeable. Bitcoins are not interchangeable as long as they have an identifiable history. And in practice participants such as Coinbase are taking that history into account in evaluating coins and users.
If Coinbase was the processor of BTC, then I would agree with you. But Coinbase is not.
The P2P and mining network is the processor of BTC. The P2P and mining network will always consider BTC to be perfectly interchangeable, 1 BTC is always equal to 1 BTC regardless of history, law or anything else. The P2P and mining network will always consider 1 whitelist BTC to be of equal value to 1 blacklist BTC, and process transactions accordingly. This maintains perfect fungibility.
If you argument is that entities (goverments) will try to make individuals value certain coins (whitelist) more over other coins (blacklist). However if they can dictate usage of bitcoin, they can dictate usage of anything, including Monero. Either way it does not matter though, to the P2P network BTC are all still identical, this is fungibility.
If Bitcoin was not perfectly fungible, then you'd have a situation were the P2P network and the blockchain processes 1 blacklist BTC as somehow equaling less than 1 whitelist BTC. That is impossible with the current network and would require a hard fork.