Sirius is a bit of an idiot
Sirius POV:
Sirius always had questions about himself, about his feelings, towards other people. Feelings and actions and impulses were just hard for him, and that seemed normal, it seemed hard for every teenager...right? Especially ones who were less sure of themselves and were only just starting to figure out their sexuality after realizing that maybe starring his friend for so long was a bit unusual. But also, who could blame him for looking at Remus so long, his beautiful eyes and don't even get Sirius started on Moony's personality.
All these feelings and impulses seemed to explode in his face one day, everything was just too much. And James was there, and he also seemed just a bit unsure of himself. It was cold out, freezing, but for just a moment neither of them felt that cold. James and Sirius closed their eyes, imagined they were somewhere else with someone else, tried to think yet not. Sirius brushing his hands along the sweater (one James stole from Remus), trying hard to imagine him instead. James envisioning not Sirius, but a different Black brother instead. Still, it wasn't very exact.
Impulsive may be one of Sirius' traits, but he wasn't going to be an idiot about this. He couldn't ruin his friendship with Remus over something stupid like feelings, especially given how deeply Sirius felt these things. With James it was easy, it was practice, both of them knew it and were fine with that.
Until they felt the presence of another person there, as they heard the door creak open they pushed each other away.
Then Sirius finally could feel that cold from the outside again, even in such a warm room and even sitting next to such a warm person, he could feel the coldness run through his soul. Even when he tried not to be, Sirius could still be such an idiot sometimes.
James coughed, and finally said, “Who’s there?”
“Is-is it Remus?” Sirius quickly added, unable to think of anyone else. Wishing with all that he could that his brain had chosen the wrong suspect and that Wormtail was just being weird.
The door closed, an invisible figure was in their room and then left. Either some stranger stole James’ cloak or one of the other boys caught him and Prongs kissing. And if one of the other boys caught him and Prongs kissing that meant there was at least a 50% chance (in his mind) that Remus caught him and Prongs kissing. Which means there’s a 50% chance that the love of his life just caught him kissing James while he thought about Remus. He didn’t say/moan Moony’s name, right?
“Prongs, who was just in here? Who was just in here?!? WE NEED TO CATCH THAT PERSON! Remus can’t have caught me kissing you, what if he gets the wrong idea, who WOULDN’T get the wrong idea? Why did we do this? Everything is terrible, FUCK.”
Sirius is abruptly stopped by James putting his hands on his mate’s shoulders, in an attempt to bring him back to reality.
“Calm down pads, calm down, we just need to find the map.”
"Is it even in here still? The person with the cloak probably has the map, it would be useless to look, only an idiot would leave the map."
James picks up the map from their shelf, "Looks like we've got an idiot on our hands...if it's Moony I'm totally telling him that you called him an idiot."
"HOW COULD YOU JOKE IN A TIME LIKE THIS PRONGS?"
"Might as well joke before you die of embarrassment," James says, "Anyways, more important, look at the name under the speeding dot on the map."
Sirius does look, and sees Remus' name, because of course it had to be Remus who was running. Remus, who barely ever runs because his hip almost always hurts. Remus who wouldn't run unless there was reason to run. Remus who probably just saw Padfoot kissing Prongs and was intensely weirded out.
The intrusive thought of punching a wall entered in Sirius' mind, that wouldn't be too difficult to fix. Other students have probably done it before. This situation is terrible enough that he almost feels the need to act out and get all that adrenaline out. He needs to get calm.
James just looks at Padfoot, visibly seeing the gears turn in his head, "Dude, we've got to go after him."
Sirius lets his head fall in his hands, and quietly speaks, "I just ruined the whole world. I can't fix this, I can't say anything to fix this."
"Pads, it will be okay, we can explain it to him, it will be fine," he goes to pat Sirius on the shoulder, trying to comfort him.
Sirius gets up, still emotional, still a bit enraged. He punches the wall, "FUCK, okay, let's go."