There are times when Leonard's brain stops forming coherent thoughts. Like an electrical circuit, it shorts out, but he has to keep talking, because that's what Sheldon needs.
At some point, what Sheldon wants and what Sheldon needs started to take priority over everything else, the way that Sheldon's restaurant schedule trumps the fact that Leonard sometimes likes a little variation in the order of his meals.
So when his brain short circuits and the only thing Leonard wants to do is to thrust against Sheldon's hand, he bites the inside of his cheek and keeps talking, for Sheldon's sake.
"The coefficient of friction can be lowered by the application of a lubricant, and the–"
"The study of friction and lubrication is also known as tribology," Sheldon says, and his eyelids flutter shut as he leans his head on Leonard's shoulder. "The name comes from the Greek root–"
Sometimes, Leonard wants to silence Sheldon with a kiss, to feel the pressure of the words that can't come out. Sheldon can't bear to be silenced, though, and the few times they tried to kiss, it was awkward.
Their noses had bumped together and it hadn't been satisfactory for either of them. Penny taught Leonard that persevering was worth it, but Sheldon is teaching him that some things can't be changed. The order of the cereal boxes on their fridge, the order in which they eat at their favorite restaurants – some things are as finite as the physical constants that govern the workings of the universe.
Leonard isn't sure he minds, either, not when Sheldon is stroking his cock, his breath coming in short little puffs against Leonard's neck and his heartbeat hammering through his thin body. Leonard can feel it, the echo of Sheldon's pulse going through his body, and he pulls Sheldon closer.
They're almost sitting on top of each other, and Leonard has stopped talking. Sheldon's face is twisted into the expression that means he doesn't like what's happening and he's going to say something that will just kill the whole situation. Leonard puts a hand on Sheldon's groin, feeling the bulge of his penis through all the layers of clothing that he wears. He's hard, and he thrusts into Leonard's hand before he remembers himself, remembers that that's one of the places that isn't supposed to be touched, and then he shifts away from Leonard.
Leonard starts talking again, and keeps talking even though he wants to close his eyes and thrust and keep thrusting. He tells Sheldon what he'd like to do, how he'd like to touch him, how he'd like to fuck him.
Long explanations are the best, and Leonard draws it out as long as he can, telling Sheldon about how he'll stretch him with one finger at first and then a second. They're neither of them experts in biology, but Leonard describes the location of the prostate gland and the sensations that Sheldon would feel when he found it.
They're both breathing hard, and Sheldon's got the look of concentration on his face that means he's about to come. Leonard puts his hand on Sheldon's, guiding his movements, and gives in to the impulse to thrust. The sight of Sheldon doing this for him, the fact that Sheldon is doing this for him…
He bites his cheek again, and focuses, and says, "Having decreased the coefficient of friction with the application of lubricant, and having loosened the muscles of your anus with my fingers, I would insert my penis–"
Sheldon's mouth forms an O, and he turns in against Leonard again, shuddering against Leonard's shoulder when he comes. This is the only time that Leonard sees him like this – collapsed in on himself, uncertain and vulnerable. Sheldon is hesitant to trust this pleasure, this biological phenomenon that doesn't appear to obey any known rules, and he needs Leonard to take him through it, to steady him.
Leonard is more than willing to do it. He presses a kiss to Sheldon's temple and puts an arm around his shoulders, holding him until his shaking stops.
Leonard's shirt is wet where Sheldon is breathing against him, his lips pressed against Leonard's shoulder in a kiss through cloth. Leonard looks down at him, at the spot of semen darkening the fabric, at the proof that Sheldon came in his pants, and that's all that it takes to send him over the edge, short-circuiting his brain at last.
Sheldon is quiet, afterwards, resting against Leonard and holding him in place with the weight of his body. Leonard puts a hand on his head, stroking his short hair, and Sheldon cuddles closer to him.
Before long, Sheldon will insist on cleaning himself, and will wash his hands enough times to be vaguely offensive – and when Leonard objects, he will cite a study on the transmission of infectious diseases through sexual contact, and that will be the end of the discussion. When Leonard tries to kiss him, he will get that funny, pinched-off look on his face that means he doesn't understand what's happening, and Leonard will pretend he was reaching past him to grab the car keys from the bowl by the door.
Leonard will drive them both to the university, and nothing more will be said about this morning. They'll eat lunch in the cafeteria, and listen to Howard's dirty jokes and Raj's complaints about working with Sheldon. Sheldon, of course, will smirk and remind them that he's on the brink of an amazing scientific breakthrough, and Leonard will kick him under the table.
It will be the same as every other day has been, since Leonard moved in with Sheldon, and yet – immeasurably different, in some way that can't be quantitative or defined. When Sheldon isn't lecturing him about communicable diseases that can arise from genital-to-oral contact, or correcting the equations on his whiteboard, Leonard thinks that it's immeasurably better. This is something he never had with Penny, these quiet moments afterward that are eventually broken by Sheldon's reflection on superlubricity or the unifying principles of tribology. This quiet understanding that they share will carry them both through the day, and to the night that they won't speak of.
Sheldon shifts against him, muttering, and Leonard holds him there for a minute longer.