It does not take Sam very long to figure out.
To be fair, however, it is in fact a gradual process. Two days after their rigorous foray into the uncharted (uncharted for Cas, anyways) world of oral sex—minus rimming because even Dean has his limits and that is one of them—Dean and Sam are sitting outside their motel room while Cas takes a shower inside, just enjoying what is actually quite a nice evening.
Dean takes in another mouthful of beer, which as it turns out is not a great idea because Sam chooses that moment to ask, "Do you think Cas is gay?"
In the moment of shock this question creates, a trickle of beer that is clearly of the opinion that Dean's esophagus is way too mainstream leaps at the opportunity to explore his trachea instead. While Sam is thumping him on the back, helpfully attempting to dislodge his lungs so Dean can cough them up properly, he adds with a tinge of disapproval, "Honestly, Dean, I know you're still not that comfortable with the whole idea of guys being gay, but—"
"Wait, what?" Dean manages to gasp.
Either Sam decides to ignore the interruption or he genuinely doesn't hear him, which is equally possible since Dean's fantastic dying whale impression is pretty damn loud. He goes on—"like, it doesn't even effect you, so why does it matter, anyways?"
"I… don't care… if people are… gay," he chokes out.
"Okay, well… good. I'm just saying. Anyways, Cas. It's not a big deal, I was just kind of wondering."
"He might be gay," Dean says cautiously, thinking that the only way Cas could possibly be more gay is if he'd started singing show-tunes while his mouth was still full of Dean's cock the other day. Which would not only be really, really gay but also extremely impressive. Unfortunately, or since Dean actually hates show-tunes fortunately, he's pretty sure the only songs Cas knows are from the classic rock albums he's been subjected to hearing over and over again as a passenger of the Impala, so his gay-ness level will probably have to rest with sucking dick and taking it up the ass.
The conversation continues for another awkward five minutes, although the awkwardness seems to go right over Sam's head, until Sam gets a phone call from Bobby with news of a prospective case. Dean breathes a sigh of relief.
Then the next day, at a rest stop on the highway as they're headed to Bobby's new case, Sam comes back from his supply run with a bottle of water for himself, a coffee for Cas who has recently discovered the joys of caffeine addiction, and a can of Purple Crush for Dean. "They were out of Coke," says Sam apologetically in response to Dean's murderous glare. "It was either this or some weird coconut-pineapple thing."
Dean pops the tab and takes a mistrustful sip. A little sickly sweet but at least it's cold, and the fizz helps block out some of the taste.
"Cough syrup?" asks Cas from the back seat.
Grinning at Cas in the rearview mirror Dean confirms, "Cough syrup—but as cough syrup goes, actually not too bad." And because he is possibly the most idiotic person on the planet he adds without thinking, "Better than those condoms, anyways."
"What?" says Sam. His expression would be comical if Dean weren't trying to locate his stomach, which seems to have disappeared as soon as he realized what he said. "Did you just say what I think you just said?"
Dean, in a remarkable demonstration of his lightning fast thinking and sharp tongue, says, "Uh…"
"What do you mean, those condoms? And how the hell would you know what a condom tastes like anyways, huh? Is there something you want to share with the class?"
"Look, it's not—" It's not what? Not an indication that Dean's been sucking guys off since he was sixteen? Not proof that the last guy on whom he performed said act is currently sitting behind them? Because it totally is. Shit. Say something, say something, just make something up… "Uh. Cas was asking about flavoured condoms the other day, and from what I've heard the grape ones are really gross."
Cas meets Dean's eyes in the rearview mirror, and thank fucking God he's getting a little bit better at this whole nonverbal communication thing because he nods his agreement vigorously. The fact that he doesn't try to say anything is also a blessing, because over the course of their acquaintance Cas has proven to be an absolutely horrendous liar when it comes to the small stuff. Big things, where your life or your friend's life or the whole world is at stake, anyone can lie about those; but it's small stuff like this, where there's always the temptation to start grinning or crack up, that's where being a good liar really counts.
"Uh huh," says Sam, still sounding skeptical. "So you just—watch that car!"
At which point Dean skids to a halt, nearly crumpling his baby's hood against the back of the silver car that has suddenly slammed on its brakes in front of them. Dean honks angrily and the driver gets out, and his black eyes and the black eyes of passengers in the cars he hasn't been paying much attention to behind him indicate they probably aren't going to get any choice in the matter of picking their next job. By the time they make it to the nearest town, bruised and bleeding and exhausted though thankfully all still more or less in one piece, the earlier matter seems to have been forgotten entirely.
Sam tries to drag Dean out to the Laundromat with him that evening, and for once his only reasoning seems to be that they both hate doing laundry so if he has to suffer through it he's damn well going to make sure Dean has to suffer through it too. Dean wouldn't have minded too much, either, except that a) it's been, like, three days, so the novelty has yet to wear off this whole arrangement he's got going with Cas and b) he just found the pack of Pop Rocks he bought the other day but never ended up eating in his pocket. So he feigns a headache, and since Cas is in the shower (again—he seems to take that old saying about cleanliness being next to godliness very, very seriously) Sam storms out on his own.
"Hey, Cas," says Dean when his friend exits the bathroom with a towel wrapped around his waist. He tries to sound casual, like the clean glow of his skin and the suggestively positioned shadows created by his hipbones and, oh yeah, the fact that he's pretty much naked aren't all huge turn-ons for him. "You ever tried Pop Rocks?"
He does that Cas-thing where his head tilts to one side as he considers the question. "No, I don't believe so."
"You want to?"
"Um, sure."
Dean rips the package open and Cas holds his hand out expectantly, but Dean just shakes his head. "I was thinking you could try them somewhere other than your mouth to start off, actually."
"I don't understand what you're—"
"Drop the towel, Cas."
His neck straightens out again, the look of confusion replaced by a grin as he does as Dean says.
It's nice, this time, to be able to take it a little slower; now that there's an agreement about this—now that Dean doesn't have to lie and bargain and trick himself into doing this—he doesn't have to plow through, or even dive straight in, because there's not fear of coming to his senses halfway through. Although, really, considering the way the rest of his life has gone he probably ought to have stopped worrying about coming to his senses years ago.
It's nice to be able to stroke lines down from Cas's hips to his groin with thumbs callused from years of hunting, nice to run his hands over the curve of his ass, nice to get him good and hard and wet and to press his lips against the base of Cas's cock before he even thinks about giving in to Cas's demands to just suck me already, please Dean; because whatever Dean may think about love and however tough he may act outside the bedroom, for him sex and intimacy are basically inextricably linked. Stupid, right? A week later he won't even remember the names of half the girls he's slept with, and a week after that he may not remember having slept with them at all. So it's just pure, physical intimacy, and even he knows in theory that physical intimacy means nothing without emotional intimacy, which is precisely what he does his best to avoid. It makes no fucking sense whatsoever—but hey, there you go. Story of Dean Winchester's life.
Even when he gives in, it's not really giving in, not the way Cas is asking for, anyways. He's still teasing—letting the underside of the angel's cock rest again his tongue without closing his mouth, licking the pre-come from his slit, moving further back to tongue gently at his balls.
Cas moans, staggering back to brace himself against the nearest wall, and tries unsuccessfully to get angry by growling, "Would you just—ah, ah. Fuck. I hate you—"
"No you don't," says Dean, nipping at his hips in a way that dissolves whatever Cas might have been about to say next into a wordless jumble of sounds. He can't help noticing that it's the first time he's ever heard the angel curse, nor can he help noticing how hot it sounds. Hot like he-could-probably-get-off-to-a-recording-of-Cas's-gravelly-voice-just-swearing-over-and-over-again hot. Though this is undoubtedly far, far better.
Okay. Enough.
He rips the candy package open and pours its contents onto his outstretched tongue (making sure Cas can see, since he's been doing this sort of thing long enough to know the importance of a good show), and tosses the empty pack carelessly aside. The candy fizzes on his tongue, a little bit sour and a little bit sweet, and finally he wraps his mouth around Castiel's cock. He can feel it popping against Cas's skin, too, and there are a lot of tastes getting mixed up in his mouth in a weird sweet-salty-sour-bitter-musky-Cas combination (good weird, he thinks, not bad weird). It's the first time he's done this without a condom—Dean figures since the world's probably going to be gone soon anyways safe sex can go fuck itself—so it's new for Cas, feeling Dean's momentarily fizzy mouth against his bare skin, but it's also a little bit new for Dean. First time he's tried the Pop Rocks thing himself, to be honest, though the idea's been kicking around for ages; and fuck does it work well, because Cas comes in, like, half the time it took Dean to get him off three days ago or whenever it was they were messing around with all those flavours. Dean barely has to do anything, just takes it until his eyes are starting to water as Cas fucks his mouth—which is good because it means he can reach into his boxers to jerk himself off at the same time. Cas seems to have unwittingly discovered the fact that having his hair tugged gets him crazy hot (one of the few times he longs for Sam's hair, actually, so that there'd be more to pull), and maybe he's always been a bit of a slut for sucking dick anyways, so there is no way he's going to wait until Cas is done, no matter how relatively quick that is.
Cas's fist slams against the wall—when Dean looks up he can see the muscles in his exposed neck standing out taught against the skin—and then he's coming, shooting down the back of Dean's throat in thick, hot spurts, and Dean's swallowing even though he doesn't have to and even though it tastes kind of strange mixed with the blue raspberry (the fuck is with that name, anyways, there's no such thing as a fucking blue raspberry). Cas sags against the wall and just fucking watches Dean finish jerking himself off, hungrily almost—they'd have another go for sure if they weren't both worrying about when Sam will be back. Still, it's nice to know Cas is into Dean for more than just the things Dean can do for him.
By the time Sam moodily stomps back in with the laundry (supposedly clean, but Dean suspects it'll smell like resentment for days), Dean and Cas are both safely in bed. Different beds, since it's Sam's turn to share with Cas again. Dean is faking sleep to go along with the whole headache story, though it only requires a minimal amount of concentration since he's almost there anyhow, and judging by the sound of Cas's breathing he's legitimately fully asleep. It means Sam doesn't have anyone to take his annoyance out on, so after a fair amount of grumbling Dean hears the springs of the other bed creak as his brother climbs in next to Cas.
The first thing Dean notices when he wakes up is his knees, in particular the fact that they feel like they're on fire. He swings his legs out of bed to examine them, and—shit. Oh, shit. Major rug-burn. Apparently blowing Cas in just his underwear was not as good an idea as it seemed at the time.
"The hell happened to your knees?" asks Sam, who is already sitting up in bed sporting a spectacular case of bed-head.
Dean tries to say something only to find his throat twinge painfully in protest—trying to deep-throat Cas, also not a great idea. Wonderful.
Sam is still waiting for an answer Dean doesn't have, so he croaks, "Uh…"
Cas, also by this time awake, catches the soreness in his friend's voice and looks horrified. Clearly he's feeling guilty—which is fine by Dean if it means Cas will be making it up to him sometime soon.
"Dude, are you sick or something? You sound—" Something in the bed crinkles loudly as Sam moves, and after a bemused moment of digging around in the sheets he produces the crumpled packet of Pop Rocks. Dean and Cas desperately avoid making eye contact with each other. "Hey, who was eating candy in my bed?"
"Uh," says Cas. "Me?"
"Yeah," Dean agrees quickly. "Him. While you were out." They both nod vigorously to support the legitimacy of the statement.
"Then how come his tongue's blue?" Sam demands, jerking his head towards Dean, who despite his best efforts with a toothbrush didn't manage to get rid of all the artificial colour that had stained his mouth the evening before.
Dean develops a sudden fascination with a just-discovered hole in his bedspread. Cas says uncertainly, "Well… um…"
Sam looks from Cas to Dean to Dean's knees to the empty package still in his hand back to Cas back to Dean, and Dean can practically smell the smoke as the firing of his synapses goes into overdrive. And then…
"Oh my God," says Sam.
"You didn't…" says Sam.
"Oh my God," Sam repeats.
"No way," says Sam.
"Cas," he says, "did you hook up with my brother?"
"Um…" says Cas, glancing pleadingly at Dean for assistance.
Dean sighs, because even if they'd had the slightest chance in hell of worming their way out of this Cas just shot it right in the foot with his atrocious lying abilities, which are apparently so abominable he doesn't even have to be actually in the process of lying for them to kick in. Basically, there's no going back now, so he might as well attempt to plow forward. "Yeah, okay? Yeah, we fucked. It's not a big deal."
"Besides, it wasn't even the first time," Cas adds helpfully, as if this somehow makes things better.
"It wasn't the—like, how many times are we talking here?"
"Four," says Cas promptly. It's at this point that Dean pretty much gives up all hope of salvaging any part of the confrontation; Cas is at his oblivious best at the moment, just dragging an unwilling Dean in his wake.
Sam's giving him a "is this really true?" look, as if he's expecting Dean to shout "surprise!" and announce this whole thing is just a setup for some reality TV show. Dean shrugs noncommittally, except that it actually is pretty committal because if any of this were untrue he'd be shouting himself hoarse trying to deny it. What exactly is he supposed to say in his defense, anyways? Explain how the first two times were just supposed to be assisted masturbation? Tell Sam how Cas gives really good head? Especially for someone who's only been doing it for, like, a week—either Dean's a good teacher or Cas is some sort of sexual prodigy, and—
Anyways.
"Okay," says Sam. "Okay. If that's what… well, it's fine."
But he still doesn't look too happy, and Dean can't figure out why. Sam is a) used to Dean having sex and b) totally cool with the idea that Cas might be gay, so what the hell is his problem?