Sam knew what Dean sounded like during sex. It wasn't something he'd intentionally come to know, but over years and years of shared motel rooms and one-night stands, Sam had heard everything. It came to a point where he made the conscious decision to not let Dean's sexcapades drive him away from whatever he was doing. Be it research, sleep or some fun of his own. Because Sam had learned from an early age that if Dean's sex life dictated where and when Sam could do things, he'd never get anything done.
So he was largely unaffected by the noises drifting through the wall of their latest motel as he was reading, although he did allow himself a satisfied grin, because the person making Dean groan and gasp in the next room was not some chick from a bar. Unless something truly unexpected had happened in the last ten minutes, Dean was currently having his world rocked by an angel of the Lord, who'd finally decided to grace them with his company after months of nothing.
About fucking time they got around to it, too.
Judging by the sounds, Cas wasn't exactly the fumbling virgin one might expect, and when Dean made a particularly high pitched noise, Sam smirked to himself. “You go, Cas.”
So far Castiel hadn't made any significant noises of his own, which Sam objectively thought was a damn shame with a voice like his, but Dean sure as hell filled the silence nicely on his own. Sam jumped slightly when there was suddenly a loud thump on the wall next to him, followed by a sound suspiciously similar to plaster crumbling. He would have been worried if it hadn't been followed up by another keening noise from Dean.
Satisfied that all was well, Sam went back to his book, largely undisturbed by the porno soundtrack next door.
“Jesus, fuck! Cas!” Dean cried behind the wall.
Sam almost laughed, because if Dean was forming actual words, then either Castiel was shitty in bed or really fucking amazing in bed. The choice of words indicated the latter, but it wasn't confirmed until a few minutes of heartfelt moaning later.
“Cas! Cas, you gotta... I'm gonna... you... fuuuuck...”
Sam actually did chuckle to himself then, because once Dean came down off his orgasm he'd probably be pissed that he didn't last more than a handful of minutes.
“Dammit, Cas,” Dean rasped, and Sam snorted into his book. But then his mirth came to a screeching halt.
“I missed you so much, you son of a bitch. Fuck, don't you ever leave me again.” Dean's voice was still scratchy, but it was all too obvious to Sam that it wasn't entirely due to all the moaning.
“I can't make that promise,” Castiel rumbled sadly, and Sam felt his gut churn in a way it never had before listening to Dean's lousy attempts at post-coital tenderness.
“Then lie! I don't fucking care. Just... tell me you won't leave. Please, Cas...”
“I won't leave you, Dean. I won't... I won't...”
Sam felt his throat tighten, and the words on the page in front of him started to blur, so he carefully shut his book, put it aside and shrugged on his jacket. He left the room quietly, and for once he felt like Dean's coping method of getting spectacularly drunk might not be the worst idea.