Remus and Sirius sexting each other and Sirius accidentally sending it to James.
That is all.
I’m sorry i cant hear you over my head canons about sirius being an adorable punk rocker in smudged eyeliner w/ piercings and moving tattoos being a cutie with remus and his oversized jumper/librarian chic
let’s be real, when james found out about remus and sirius circa sixth or seventh year after they’d been together just a short while he probably looked at them all confused and was like, “wait, you mean you HAVEN’T been together since third year???”