Ahh, anon asked for a OP or Megatron guilty wank.
Here’s the Optimus version I had but separated out since it ties into Thundercracker with a Con!SO. In here, the Autobots managed to get the Reader in their later stages on carrying a trine.
Optimus doesn’t know how he managed to get into the sealed off unit in the medbay, nestled near Ratchet’s high-priority patient.
If you were asleep, he could fight the Matrix’s influence. Force himself away and take on the howling barrage. But you're awake, delirious from the stupor and possibly confused where you're at, but awake, moaning and trying to turn towards him.
Optimus immediately puts a stop to that. Pinning your arms away and keeping you on your side. Your back flushed against his front, and you’re so very warm. The Matrix hisses, disliking the action; it wants to have the carrier underneath, feel the press of a very swollen abdomen full of new life after so, so long.
(He knows if he presses against that swell, he won’t be able to let you go. The dormant protocols and Matrix will latch onto you in a hyperfixation and force him to remain with the newsparks until they’re mechlings.)
You’re grinding against him, cover already open and dripping with tantalizing heat. His panel clicks open without his permission and he can’t command it to shut. His own frame betrays him as it falls under the Matrix's overwhelming desires, the protocols flare to life as he catches a warm, delicious scent, immediately hounding him to relieve your poor state. You cry, crackling with charge as his spike extends slowly, its ridges catching over slick, quivering mesh.
Your frame’s desperate; its heightened metabolism completely outpaces the supplemental infusions. That method could work with just one newspark forging, not three devouring all the materials.
It desperately needs transfluid, and you're already developing signs of an impending heat to attract donors. He can’t fight both the protocols and the Matrix’s obsession on those newsparks, especially with you rubbing against him. He has to compromise or he’ll go into a rut.
Optimus inhales the heat pheromones, groaning and trying to keep himself in check. He shifts to hold your arms away with one hand, and you shiver as he slowly works his way inside, rocking to push the tight rings apart, keeping you in place as the protocols surge hot and hungry, the Matrix not far behind. Givegivegive, it demands, chanting over and over zealously.
It matters not that you're taken. That you're a Decepticon. That you have a partner that’s part of the Command Trine. All it cares for is helping the forging process on the only carrier it's been in contact after four million years of inactivity. All it's focused on is that a carrier is alone, a carrier that is in need.
And needy you are. You whine, struggling against his firm grip, trying to shove back and force a faster pace. You overload quickly, valve clenching tight and he follows with a sigh, engines purring.
You gasp, optics far too bright but still coherent enough to say his name and beg for more.
The Matrix burns and the protocols go rabid and his tenuous hold collapses.
Optimus onlines with a shout, vents gasping as steam escapes in heavy bursts. To his later shame, he catches the remaining wisps of the induced dream, back into a tight, needy valve clenching and fluttering and pressed against a heated, sparked frame.
His own overload leaves his lines viciously burning, the charge had been held on for far too long. His own spark aches, yearning fiercely. The Matrix's influence bleeding into his sidelined want for a sparkling.
Its croons take a darker, insidious turn when it tries to latch on and twist his soft wish: his lap full of little, chirping Seekers and you sleeping peacefully in his berth, hands over a swelling belly and-
Optimus stumbles over to his private washrack, spike still pressurized and throbbing, operating as if he's actually with a carrier in heat, refractory period nonexistent. Under the spray of cold water and braced against the wall, he fists himself harshly in an attempt to use pain to cool the active protocols.
It doesn’t. His servo is a poor substitute, so he uses the lingering sensations from the dream. You underneath and holding on his sides, legs wrapped around his waist, that swell pressed hotly against him. Optimus falls into it easily; engines roaring, charge racketing fast, the Matrix and protocols purring and possessively pleased: Keep you stuffed. Keep you sated. Keep you here.
He keeps it up until he’s finally spent, spike could only twitch and dribble a few droplets, and the cold water registers on his sensors. The Matrix murmurs angrily at the waste, protocols hissing in agreement. The transfluid all over the wall and floor is far thicker and darker, heavy with nanites.
But he could only picture you sleepily content, thighs wet and splattered pink, newsparks kicking and following his digits.