"For a man, a plutonic friend is a woman you was tryin' to fuck, made a wrong turn somewhere, and ended up in the 'friend-zone'. Oh no! I'm in the friend-zone!"--Chris Rock
When you have a platonic sexual relationship but you know that the "just friends" part was never a real thing in the first place. You pretend like it exists, but it was always a myth, much like the planet Pluto.
JB: Let's be strictly plutonic friends who have sex whenever we feel like it.
Eris: Sounds great! I love pretending, so that will be fun.
JB: Great, can you give me a plutonic blow job?