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RULE #1: He knows immediately that Sirius is trouble.
Every time naturally talented and clever af Mr. Black raises his hand in class, so eager for Remus to just look at him, to give him any microscopic form of attention, Remus's cock stirs and his first thought of the day is, "I will have this boy writhing on my cock by the end of term."
RULE #2: The outfits get sluttier the longer Remus keeps it professional.
But not because Remus avoids rule-breaking. He hates the system, the system can go fuck itself—he's the type of teacher to wave an entire week's worth of classes to give a student with burnout a break, fuck the soulless rigidity of academia. If they care, they'll catch up, and they always do because he's damn good at his job.
But. He loves watching Sirius try to seduce him.
At night, his hand drifts towards his hard cock because he caught sight of something red and was reminded of Sirius. Like a psychic, he knows that Sirius is trying on little outfit after little outfit somewhere across town just for him. How the boy is carefully picking out the colour of his thong like that will be the final straw—the final push his Professor needs to fuck him in the lecture hall after class.
And Remus knows this because he intentionally stares a little longer at the thin straps when they're red, just so that Sirius will catch him.
And proving his hypothesis correct, the following week it's nothing but maroon thongs, scarlet, crimson, burgundy—and Remus spills cum down his cock, making his fingers sticky and slick enough to keep going until it hurts a little.
He adores the fact that Sirius is trying so hard to please. And also just what a good and perfectly fuckable slut Sirius will be for Remus once he finally gets to play with him
RULE #3: He knows he's torturing Sirius, but Remus has always been a sadistic man.
But first he wants Sirius to be desperate for it, to be so thirsty for even a drop of validation and attention that his cock will harden just with Remus’s smile. And if the way Sirius squirms in his seat every time Remus's eyes linger on him while lecturing means anything, it won't be long now.
RULE #4: Sirius always thinks he's the one doing the seducing until Remus declines his drink and says casually, "Let me show you something, Mr. Black..."
And it's not a quiet spot for intellectual conversation. It's not an intimate booth where Sirius divulges his trauma. Remus yanks the boy straight into the public toilet and pushes Sirius to his knees until there's a cock in front of his face and Professor Lupin dipping his thumb inside Sirius's mouth, tracing his teeth, caressing his tongue, stretching one side of his lips wide just to show he can.
"Be a good boy now, Sirius, and open that clever mouth for me," he says as he presses the head of his cock to Sirius's pretty lips. And Sirius eats up the praise and immediately commits to trying to be the best blow job of his Professor's life.
Remus shoves the entirety of his cock as deep into Sirius's throat as he can. He makes it his life's mission to fuck the boy hoarse so that the next day when he calls on Sirius in class, relentlessly, for almost every question, Sirius will feel it and remember the rough fuck in his mouth from the night before.
And also remember the way Remus's thick milky cum coated and soothed the rawness after Sirius licked and swallowed every last drop like a gift.
RULE #5: Remus is not soft the first time he fucks Sirius.
There are no satin sheets. No candlelight. No foreplay. There is a mash of lips and smearing of Sirius's lipstick. There's tearing Sirius's clothes, stealing his little red thong for later, and lightly spanking Sirius's arse until Sirius asks shyly, "Can—can you do that again? Please?"
"You like me spanking your arse?"
Slap. And Sirius shudders.
"You want me to make it so nice and pink, so pretty for me to fuck?"
Sirius moans and nods his head, arching his back with the grace of a natural whore.
Slap.
"Ah!" Sirius screams and moans and takes in shaky breaths as his arse begins to sting, the idea of being spanked by his Professor enough to make him come unfucked, untouched, desperate and writhing. "More!"
"More what? More punishment? Because Daddy only fucks good sluts. Don't you want to be Daddy's? My pretty baby who takes my cock so good in his tight little hole?"
Sirius bites his lip and relents, pleading, "Please, please be my Daddy. I'll... I'll be good—I'll be so good for you.”
Remus spanks Sirius until he's a drooling mess, and when he finally thrusts into him, he makes sure it's deep enough that he can slam his hips and make Sirius's pink arse sting with each slap of skin.
RULE #6: Remus will always—always—do anything he can to keep Sirius in his bed, and that includes offering an extra credit point for every time the boy moans that he's "Daddy's little whore," or holds his arse open for Remus to spit on his hole, or takes Remus's cock a little deeper in his throat.
And Remus keeps count of these favours.
And he does award them under 'Participation' in his grade-book.
In the mornings when Sirius is trying to sneak away from his Professor's bed, Remus will pull him back by his waist and grind his aching cock against Sirius's arse.
"I have to go," Sirius will protest breathlessly, trapped helplessly on his side in Remus's arm and listening with anticipation to the bottle of lube closing behind him.
And Remus should respond by pressing his slippery cock along the cleft of his arse, sliding deeper with each teasing thrust between Sirius's cheeks. He keeps it rhythmic until his boy's mouth waters for those thrusts to be in him.
"I have to go to the library,” Sirius still protests.
"Fuck the library," Remus almost growls and catches the tip of his cock on Sirius's eager hole, applying just the smallest of pressure against the tight ring.
"Professor Lupin," Sirius teases, but swallows a moment later when Remus's hips stutter at the name, pressing into him a little deeper. "I—I have to study so I can start my assignment for your cla—Ahh—" Sirius's mouth drops open, Remus pushing the tip of his cock slowly against the tight pink ring, rocking his hips against Sirius's arse, working in one inch at a time until all Sirius knows in life is the drag of Remus’s cock.
"I'll quiz you," Remus teases lowly into Sirius's ear. He pushes his hips flush against Sirius's arse, savouring the tightness of his boy's hole as he adjusts to his Professor's size.
But Sirius can never think like this, his head is always floating somewhere above him as his Professor grips his fingers around his throat. He's not allowed to breathe until it's his turn to answer, but he can't even remember the question.
Again, all he knows is that there's a thick cock stretching his arse open at seven in the morning. There's a hand on his own cock, running wet circles along the tip with his own beads of pre-cum, just teasingly enough to keep his orgasm on edge while his Professor starts really fucking him.
"What's wrong, baby? Can't think?" Remus will taunt.
And Sirius will whimper.
"Daddy's dumb slut. Can't think when there's a cock in him. Not even enough to answer a simple question."
He starts fucking Sirius's poor hole harder, choking his delicate throat tighter.
"You're going to end up needing a lot of extra credit to keep up your grades, baby."
Then Remus leans in and whispers.
"But if you let Daddy come in your hole instead of pulling out, I'll give you full marks."
Sirius just squeezes his eyes shut and nods.
Remus lets go of his throat, pushes Sirius over onto his stomach, and straddles his arse, deep long thrusts that turn to harsher smacks of skin. He pounds into his boy, angling just right so he can hit the spot that turns Sirius into a drooling whore.
And when Remus's hips stutter and he comes in Sirius's arse for the first time since they started fucking, all he can think is:
“I’m never going to let this end.”
Remus slips out, a trail of cum falling out of Sirius's gaping hole along with it.
And Professor Lupin spreads his student's arse cheeks apart to admire the contrast, of his white sticky cum on his boy's slick, glistening pink hole.
And Professor Lupin will look up and smirk. "Full credit, Mr. Black."
…But this is just a guideline. 😇