After the previous incident at the motel, Sam had insisted on getting separate rooms. Dean was confused about it, but he didn't argue. That's how he and Cas ended up sharing a room.
At exactly 3 AM, Sam Winchester woke up, startled. Disturbing sounds were coming from the next room, where his older brother and their ex angelic friend were sleeping. Well, should have been sleeping, anyway.
Sam, his sleepiness having left him entirely, sat up in his bed and listened.
Someone panted heavily and groaned.
"Dean… Dean I can't."
"Yes you can, come on Cas!" Sam heard his brother growling.
More panting and shuffling.
"You aren't doing it right, Cas!"
"Really? I think you are doing it wrong!"
"Shut up, Cas!"
Sam strained his ears to hear better as something rustled and hit the floor. "Clothes. They're naked!" Sam's mind was imagining disturbing images of his brother and the angel in obscene poses, and it was disgusting, but he couldn't stop listening.
"Dean, why won't you listen to me?"
"I am listening, feathery brains!"
"Then push harder!"
Sam's face turned beetroot red and he buried it into his hands.
"I am pushing, Cas! I'm PU-SHING! How much harder do you want me to push?"
And that's when something banged against the wall that was separating their rooms.
"Yes, finally," Castiel moaned.
"Yeah, babe! There, that's it! That's it!" Dean yelled happily and the banging repeated.
Sam could not take it anymore. The images of naked Dean and Castiel doing dirty things just raped his mind and he jumped out of his bed, dressing hurriedly and running outside of the motel to sleep in the Impala.
Meanwhile Castiel helped Dean move the hunter's heavy bed away from the window, from which the cold air was sneaking into the room, disturbing Dean's sleep.
"Thanks, Cas," Dean grinned at his friend before crawling into bed.
"But you're still a baby, now without a trench coat," the hunter smacked Castiel's ass and ducked under the covers, laughing.