Remus is achingly, painfully hard, and it's all Sirius's damn fault, as usual. The git had the gall to pull Remus aside in a corridor, pull him out of his way, make him late for Transfiguration, and still neglect to finish the job. Snogging and grinding and a few quick strokes, and then Sirius's head had snapped up, away from the raw wet redness of Remus's neck, some phantom echo of talking spooked him, he fled because, as he's so fond of reminding Remus, it's just sex, our little secret, nothing gay about it.
And so Remus has to stay in this corridor, thinking about anything but Sirius's mouth and teeth and cock and hands, thinking about anything but the nips Sirius gives when they're in the midst of it, thinking about anything but the moans and whispers and hissing and gasps and all the fuckinggorgeous sounds Sirius makes. Anything but that.
Meanwhile Sirius will go about his merry goddamn way, laughing and joking and pulling pranks and pointedly not touching James because everyone with eyes knows that if Sirius wasn't so stupidly tied to the Black family lineage (Remus knows enough to know that Sirius might have run away but there are some things you can't ever get off your back, even when you don't believe them anymore) and if James wasn't strangling himself with love over that redheaded Evans girl, then it'd be James Sirius pulled into a corridor for a quickie, not Remus. Everyone knows that.
And so sometimes, when they think no one is looking, people are quietly kind to Remus. Because anyone with eyes can see how he tunes himself to Sirius, how he pointedly doesn't do any of the myriad things he could do to force Sirius to treat him better, how he lets Sirius take and take and take and never demands anything in return, not even a cigarette or a fucking kind word, nothing.
Soon enough — although not so soon as to keep him from losing five points for Gryffindor for tardiness — it diminishes, and Remus is able to walk steadily and swiftly down to the classroom, and the fact that Sirius sits huddled with James and doesn't even notice Remus's entrance, well, that's par for the course, isn't it?