Dean wakes up in the middle of the night and he can still remember everything about his dream; Castiel rutting against him, the way his mouth was clashing against his, the way Dean was moaning and begging Cas for more... Dean remembers all of it, and the way his cock is behaving, Dean bets little Dean remembers as well.
Dean looks to his left to check on Sam, they're in a motel somewhere near North Carolina and Dean knows that Sam is sleeping way too deep to be awaken with low moans. Dean breathes deeply and grabs his cock, he give it a few strokes and then stops himself. If he's going to do this, he will have to do it right.
So this is how Dean finds himself grabbing the bag beside his bed and grab a big pink dildo and lube from the bundle of clothes he has mixed up in there. The plan, if Sam ever found them, was that he would simply say it was supposed to be a joke for when Sam had lady guests in their motel room.
So here is Dean, fingering himself open, wishing for Castiel to be there and to do the thing for him. He moans silently against his pillow, his butt is up, while his face is down on the pillows, making it as easy for his thick fingers to slide inside his body. As he gets to the third finger, Dean decides to search for his prostate and it takes a while, but when he does, Dean has to bite his tongue in order to stay quiet.
Soon, Dean can't take it anymore and puts some lube on his pink dildo and strokes it, Dean remembers doing it in his dream, he remembers doing it to Cas, and he certainly cannot stop thinking about doing it to Cas now. He wonders if Cas can see these images in his head, because what Dean is doing could be called a prayer; he is wishing for Cas's presence and showing him all the dirty thoughts in his mind right now. Dean can't even shield his mind, he can only think about getting that dildo into his body, so that's exactly what he does.
Carefully, Dean slides it in, giving himself a few seconds to adjust to the size and after a few seconds of agonizing need to continue, he does so. Dean slides the dildo in and out, his rhythm alternates between hard and fast strokes to slow and deep ones, at some point, Dean can't stop thinking about how Cas would fit perfectly inside him and his mouth starts calling his name, his voice is a broken sound, too small to be hear by Sam, but loud enough for Cas's mind to find him.
"Dean." Cas says as he looks down at Dean's naked form.
Dean isn't sure if he should stop everything and hide his body or moan at the sound of Cas's voice. He ends up being too consumed by lust to be able to stop and his mouth simply moans and his breathing hitches as he tries to reach deeper into his body. Dean isn't even sure if Castiel is actually there or if it's simply his mind providing really good help at this hard task.
He believes Cas is real the second he feels Castiel touch his spine with his calloused fingers. He believes Cas is real the second Cas's lips trail down Dean's spine until they find Dean's bottom. He believes Cas is real when Cas's fingers take the dildo from his ass and his mouth starts fucking his hole.
Dean's voice is gone, he cannot say anything in fear of waking Sam up, so he uses his hands to grab Cas's face as close to his body as possible (obviously it being a difficult task, since he is stomach down and his arms have a slower reach in that position).
Cas' tongue does its best to fuck Dean's bottom and he loves it when Dean writhes beneath him. Quickly Cas finds himself taking his pants off and Dean reminds him to put on a condom and spread some lube on himself first.
When everything is ready, Cas slams into Dean, it isn't soft, it isn't careful, but it's exactly what Dean needs and wants. Dean needs to be fucked hard and fast against the mattress, Cas obliges to those unspoken wishes from the back of Dean's mind and soon enough, Cas's breathing is just as uneven as Dean's and he has a difficulty at keeping his pace. When he just can't control himself, he simply thrusts in with as much strength as he can, angling his cock in order to hit Dean's sweet spot over and over again, leaving Dean babbling, absolutely unable to control his mouth from yelling "HA-AHHH.. HARDER" at each thrust of Cas's hips.
And that is exactly how Sam wakes up, to the sounds of his brother being fucked in the butt, yelling like the needy guy that he is to be fucked harder. Sam's eyes go wide and he gets up as fast as he can and runs away from the room, into the car.
Dean cums not long after, not even noticing that Sam is gone, and his shouts are loud, letting the world know exactly how well fucked he was and how he does absolutely not give a fuck if everyone else is trying to sleep. Cas cums as well when Dean's butt starts contracting its muscles, leaving Castiel unable to speak.
Cas stays over that night, Dean falls asleep in his arms and for the first time in a long time, he feels safe. The next morning is slightly weird, with Sam blushing whenever he sees Dean and absolutely refusing to look at his face for more than one second.
Cas stays over every night after this, Sam and Dean start getting separate rooms and Cas fucks Dean whenever it's possible, whenever Dean begs him to and whenever he wants to because he just knows how into it Dean is as well.