A/N: I didn't write it down before, but you can also find this work on AO3 under the same username.
Snape picked him up and carried him to his desk, shoving the papers to the ground and sprawling his form over the wood. He kissed him again, looking deep into his eyes.
"Answer me truthfully: are you a virgin?"
Harry shook his head, even though he was scared of being less desirable for that, but Snape only nodded in acknowledgement and proceeded to remove his clothes. When Snape's fingers touched his bare skin, he felt tingles descending through his nerves.
It didn't take long for Harry to be completely naked over the desk, while Snape loomed over him still fully dressed. It made Harry feel vulnerable and small; his dick throbbed harder at that feeling.
Snape trailed his fingers over his chest and descended across his belly, circling around his dick and grazing his crack. Harry let out a shaky moan.
"Y-you're teasing me, sir," he managed to say.
Snape chuckled. "Mr. Potter, I've barely started." And then Snape got down on his knees and licked Harry's arsehole.
Harry's entire body arched, mouth opening in a silent moan. That was filthy, forbidden, and absolutely delicious. Snape's tongue was hot against his opening, hands resting on his thighs to keep them spread. Harry gripped the edges of the table, feeling the uncontrollable need to talk.
"T-that's so good, oh fuck, Professor Snape…" Harry heard his partner's breath hitching when he called him by his title and smirked. So they shared the same kinks. "Professor, I've been such a naughty boy, don't you think I deserve some punishment?"
Snape groaned, the puff of breath hitting Harry's arse and making him shiver again, a filthy moan escaping through his lips. He heard him muttering some spells, but couldn't understand them.
"What have you done to deserve punishment, Mr. Potter?"
"I have lu- oh. OH." A finger coated with lube suddenly penetrated him and his hips bucked forwards, trying to get more.
"You were saying?" Snape said in a breathy voice, the only indicator that he was as turned on as Harry was.
"I have… I have…" He had lost track of what he was saying. "I have lusted over my professor," he finally managed to say.
A second finger joined the first and Harry's eyes rolled into the back of his head. He felt so exposed and the slight brush of Snape's clothes only served to turn him on more. The movements inside of him were slow and torturing, he tried to rock his hips forwards to get more but Snape held him back.
"Hmm. I think I know exactly what your punishment is going to be," he said and removed both of his fingers. Harry whined at the sudden emptiness.
"Come here," Snape said, sitting on his chair. "Kneel in front of me."
"Yes, sir."
Harry's eyes lit up with the prospect of sucking him off. He kneeled in front of his professor, bum resting over his heels, hard dick grazing Snape's trousers lightly.
He didn't waste any time unbuckling his belt and freeing his professor's dick from his pants. His breath hitched at the sight of the ridiculous long dick with an impressive girth, already rock hard. He traced the veins with his fingers. It was only the second cock he had seen in his life besides his own, and he wanted to savour the moment.
He gave an experimental lick over the length, not taking his eyes off the man in front of him. Snape's breath was laboured, but he didn't close his eyes, not even when Harry opened his mouth and swallowed him up until he choked on the length. He wasn't very experienced, but he bobbed his head up and down at a steady pace, running his tongue over the length and making sure to avoid his teeth.
Severus couldn't believe his eyes. He honestly hadn't considered this outcome when he recognized his feelings for Harry. Now the boy — no, the man was kneeling in front of him, naked, and Severus watched as in a trance while his dick disappeared and reappeared from that hot and delicious mouth, his eyes never straying away from him, shy hands touching his balls and thighs.
He tugged on Harry's hair experimentally and the man shivered under his hands, his eyelids fluttering with pleasure.
"I will fuck you now," he said, and Harry removed his mouth from his dick with a popping sound. He leaned over the table, chest and face touching the wood, arms stretched forwards so he could grip the edges. Harry spread his legs and tilted his arse upwards. Severus groaned at the sight.
"Is that how you fantasized us fucking, Mr. Potter?"
Harry moaned. "Y-yes, sir."
Again, Severus coated his fingers with a lube spell, but he didn't need to do the cleaning charm again. Harry's hole was already fluttering with the desire of being filled, but first Snape had to make sure he was well stretched. He might not be a virgin, but he was still bloody tight.
One finger made Harry moan, two left him shaking, and three made him buck against him forwards.
"Please, Professor Snape. I need you. I need you to fuck me."
That filthy mouth was going to make him lose it. He removed the three fingers and spread lube over his cock, lining it up against Harry's entrance. He waited for a second, trying to control his breath and his mind so he wouldn't come in two minutes, and then slid it inside.
"Oh, Merlin. OH, that's it."
Severus went slow, for Harry's sake and also for his own. The sight of his dick being swallowed up by that arse was already tempting enough. Even trying to be careful, it seemed that no time had passed until he was balls deep, the fabric of his clothing rasping against Harry's round cheeks.
He scraped his nails against Harry's back, dick pulsating with the need to move, but he waited. Harry didn't seem to like the waiting because soon he was wiggling against him.
"Please, Professor Snape. Please, fuck me."
He couldn't hold it any longer. Severus pulled almost all of his length out and then slammed it back in; Harry whined in pleasure. He held the slim waist and pounded mercilessly against Harry's arse, relishing himself on the filthy sounds of slapping skin and those delicious moans his partner was making.
The feeling of a dick sliding through his cheeks wasn't new, but it was still breathtaking. He would die happy with that full feeling in his arse. The slight burn of the stretch wasn't off-putting; it was quite the opposite, it only served to increase the lust haze on his mind.
When Snape began pounding him hard, he lost it. He was saying something, but he had no idea what; the only thing he knew was that he was asking for more. Snape obliged, pounding harder and faster until Harry couldn't speak any more.
He was overwhelmed by the sounds: the squelch of the lube, the slapping of skin against skin every time Snape thrust inside him, the creaking of the desk below them, and the distant sounds of people going to dinner, unaware that a student was being thoroughly fucked by their Potions Master.
He was feeling so much at the same time. His throbbing cock, pleading for attention, rasping against the wood. His bare chest over the cool surface, sweat dripping on his body, legs bouncing with every impact… He could feel Snape's hands on his hips, holding him so firmly that he knew he would be bruised after, his balls hitting against Harry, his length all up inside Harry's arse, stretching him, filling him…
He was lost, completely lost, incapable of giving his dick the attention it was asking for. Harry only let himself be used, and it was the best sensation he had ever had.
Snape stopped pounding onto him, but before Harry could ask what happened, he turned Harry over on the table. Now his back was laying flat against the wood and he could see Snape's face clearly. He was still almost completely dressed and that sight only could have made Harry cum, but Snape held the base of his dick.
"You'll cum only when I let you," Snape said.
"Yes, Professor Snape," he said breathlessly, making Snape groan again. His eyes flashed with malice.
Without removing his dick from Harry's arsehole, he pulled Harry's legs upwards, resting them over his shoulders. Snape gripped the underside of Harry's thighs for leverage and resumed the pounding.
Harry was already overstimulated, the situation he had fantasized about in every Potions class was happening - the man he liked was pounding his thick dick inside him, his own cock slapping against his belly with every thrust. He thought he couldn't possibly feel better, but then he felt his teacher's dick pressing something inside of him that made him scream. He gripped his own dick tightly to stop himself from cumming.
That was... He didn't have words.
It hit him again in that sweet spot and Harry cried out with the need to cum, and he looked at Professor Snape, too lust-crazed to talk, but his desperation said everything.
"Cum for me, Harry," Snape said, hitting that delicious spot again.
Harry's orgasm hit him like a wave of heat. His voice faltered and his entire body shook, brain short-circuiting with the intense pleasure he was feeling. The heat was so intense that he barely felt his cum splattering over his belly before his body went limp.
Snape wasn't able to hold it much longer and soon he came with a low groan, coating Harry's insides with his release. Spunk leaked out of Harry's abused hole when Snape removed his dick. They were both panting, but Snape still managed to help Harry to stand up and then snogged him senseless, ignoring how Harry's cum stained his clothes.
Harry couldn't stop smiling. "I look forward to doing it again."
"You'll be the death of me," Snape said, but he was smiling.