It's very simple.
Biologically speaking, it's a bad idea because breeding from genes too closely related leads to anomalies, problems, throw-backs.
Little chance of that happening here.
Psychologically speaking, it's a bad idea because it connotes an oddity in the relationship, a deviation from the normal.
If they were worried about normal, they wouldn't be here. They wouldn't be them.
In any case, the problem's already there, inside his head. He's odd. He's deviated.
There's no sin in the act that hasn't already been committed a dozen times in thought.
Sam has this dream - recurring - where he's fucking his brother.
He never had them when he was still living with Dean and Dad - thank Christ. It was sort of... well, not fine, but manageable when he was sleeping next to Jess, could roll over and bury himself in her soft, curved warmth, the cherry-vanilla small of her. But now that he's travelling with Dean again, the dreams aren't actually showing any signs of going away.
He's never actually done it with a guy. Not it, not all the way. He's not gay. There had been that one guy at a party in his first year. He'd had broad shoulders and short hair and a bottle of tequila. Sam doesn't quite remember why he said yes when the guy said, "Let's get out of here," but he did.
They'd gone down the beach and Sam jerked him off with sand down the back of his jeans and tequila vicious under his tongue, and then he discovered that a blowjob from a guy was, actually, pretty different from one given by a girl. Confidence with the equipment, maybe.
He saw that guy around, sometimes. They never said anything more than, "Hi."
But that's the limit of his experience with guys. Which is why the dream bothers him - well, one of the reasons - because it's so vivid. It's a pretty tight focus; Sam isn't sure where they are, or even where all their limbs are, but it's got every tiny detail. Dean's sweat on his tongue and under his fingernails. The feel of his hipbones under Sam's thumbs. The hair on Dean's thighs, the double-time pant of Sam's breath, the splay of his knees, the urge to sink teeth into that shoulder. And there's a mirror, or it's some weird dream thing, because he can see Dean's face as he, as he, as he... and it's twisted and elevated and so fucking open as he spits, "Fuck, Sam, yes."
Sam wakes up gasping this time and Dean's leaning over him in the motel-room dark, saying, "Bro, the fuck?"
He's reaching for Sam's shoulder, and Sam flinches away, knocks his wrist aside. Dean recovers, smacks his arm, and Sam pushes him, so Dean pushes him, until Sam shoves so hard Dean has to take a step backwards, and Sam rolls over onto his side, turning his back.
"Fucking fruitcake," Dean mutters, grumpy as a woken bear, as he staggers into the bathroom.
Sam's still breathing heavy, utterly awake in the dark, and so hard he could cry.
In the morning, Dean's bitching about some of us needing our sleep and freaks waking him up in the middle of the night, so Sam drives the first few hours. It's silent except for Dean's occasional snore, and it feels kinda weird. That there could ever be this much silence when the two of them are involved.
They used to make a point about it, the noise thing, and Dad was happy to shunt them off into their own room as soon as they were old enough that hotel management went for it. It'd been around then that Sam had been falling asleep one night when he heard noises from Dean's side of the room that made him frown, then roll over and hiss,
"Dude, are you jerking off?"
"So?" Dean huffed.
Sam threw a pillow at him. "A little fucking discretion!"
Dean just laughed, and the noises didn't stop, the rustle of linen, the faint squeak of one solitary bedspring. "Like I don't know what you're doing when you take half an hour in the bathroom."
"You go outside."
Dean laughed again, a little faint and distracted, and Sam rolled back over, shoving his head under the one remaining pillow. It didn't help at all block out Dean's breath getting shallow, then hitching, and the breathy grunt he made.
Next morning, Sam left the bathroom door half an inch ajar, and jerked off in the shower loud as he pleased. Groaned under the spray as he came, and grinned at Dean later, strolling out.
"You about fucking done?" Dean demanded, shoving past.
Revenge came two nights later. Dean had gone out - was legally able to and Dad had long since stopped trying to prevent it - and Sam only half woke when he heard the door open. Was about to roll over and go back to sleep when he realised Dean wasn't alone.
A giggle - a female giggle - and somewhat unsteady footsteps. Sam rolled onto his back carefully. Could only see them in black-on-shadow silhouette. The girl wasn't wearing much, and jangled. Dean muttered something. She said, "Hey, c'mon now, let's get to this. I'm a busy girl." And laughed as she pushed Dean down to sit on the end of his bed, went on her knees in front of him.
A hooker. Sam couldn't quite believe it. Dad'd kill him. Dean was touching her - maybe his hand in her hair - as she unzipped his jeans, yanked them down. Dean took a quick, sharp breath. At that stage, Sam was all of fifteen and he'd never had his dick sucked, but by the sounds Dean was making, it was a pretty grand experience. Little twitchy "ah" noises and hums like groans. And then he started talking; "oh yeah. like that. fuck yes."
It didn't take long. It felt like forever. And then Dean was making a noise with more consonants than vowels in it and about five seconds later the hooker was standing up and straightening her skirt and hair. Dean reached for his pants.
"Shit," she said suddenly. "Is that-- is someone else here?"
"My brother," Dean said, passing her money.
"What if he'd woken up? You're a noisy bastard."
"You could've done him too." Cheekiness in Dean's voice, easy to imagine his grin. "I'd pay."
For a second, Sam had been tempted to sit up. Say, "Come on, then." Slide over to the edge of his bed and let her kneel between his thighs. Make Dean turn on the light so he could watch him watching Sam get blown by the hooker he was paying for.
Dean wakes up around ten thirty with his usual grizzling and stretching and general carrying on. Sam's already pulling over before he says, "Give it up, sunshine."
As they're settling back into the car and Dean's fixing the seat and the wheel and the rearview mirror like the prissy bitch he is about his car, he says, "That didn't sound like the same old nightmares last night."
It's unexpected, and Sam knocks his knee against the dash, hard. "Fuck. No. Ow. It was nothing." Dean just gives him a look before gunning the engine. "Nothing, man. You startled me, with the looming."
"Whatever," Dean says, reaching for the radio, and the silence is over.
Dean sleeps on his stomach; Sam knows this. He just sleeps in his boxers, and Sam knows that too.
It's hideously hot tonight, and Sam's just woken up from the dream. Silently, though he feels like gasping, and he's sitting up in bed, looking across to where Dean's sprawled out on his stomach, covers kicked off, limbs everywhere. The curtains aren't thick enough to hold back the moonlight. It stops short of Dean; reflected off the sheets it crisps his skin.
Sam's standing before he realises it and certainly before he has time to think. As soon as his knee touches the mattress, barely any of his weight behind it, Dean's hand goes for the pillow - for the knife under it - and Sam moves faster than he knew he could, catching Dean's wrist just short, and then the other. He has to climb on the bed fully and straddle Dean for balance, pressing his wrists down, pressing his body down and whispering, "Shh, it's me," into the back of Dean's neck.
Dean stills, and Sam's still saying, "It's me, it's me." He's pressed all along his brother's back and this is a problem because his boxers and Dean's are doing nothing to disguise Sam's erection against the curve of Dean's arse. He even smells the way he does in the dream, the musk of sweat, sour from sleep.
Sam can't help it. He pushes. His hips against Dean's, Dean's against the mattress. He's breathing through his mouth in the space between Dean's shoulder blades, his arms braced over Dean's. Dean shifts, presses up, tilts his head a little. It bares his neck and Sam bites him there, like he always wants to in the dream, even as he pushes again, pushes Dean down. Dean makes a noise that isn't quite anything, and Sam pushes again, harder. Realises that there's something that could fall into rhythm here, him humping Dean humping the mattress, and he does it anyway.
He does it, his lips against Dean's shoulder where he's bitten, breath against the saliva he's left. He shifts his hips, so he's grinding Dean down with only an oblique slip-bump of his cock against Dean's arse. Dean breathes quick and shallow against the sheet. Sam keeps his hands around Dean's wrists, because if he lets go, if he even pushes them together into one grip, gets a hand free, he'll do something like press it to Dean's hip beneath his, push it beneath them both to feel Dean thrust (being thrust) against his palm. He doesn't, he can't, and Dean, in return, doesn't give him a thing but his mouth open and damp against the bed, and a judder in his hips beneath Sam's, a twitch of his shoulder.
Sam peels himself off Dean's back carefully. Shifts to the side. Lets his wrists go last, turning away. He's facing the window, and as the bathroom door slams shut, Sam realises that it wasn't moonlight, all along, but the slowly warming light of dawn.
He drags a t-shirt on, pulls on his jeans. He's still hard, and fastening them isn't the least painful experience of his life. He goes out. The heat's left the night, and dawn is chill enough to cool him down on the walk.
When he gets back, Dean's lacing his boots. Gives Sam a single glance as he walks in. "Coffee," Sam says, holding out the cup.
Dean finishes and sits up. Looks at Sam, hands on his knees and his eyes hard, but Sam knows how to make his eyes hard too, and when it comes to stubbornness in this family, Sam is the reigning champion. Dean takes the coffee.
"We need to talk," he says, and Sam doesn't flinch, "to that girl about her father's collection. Talisman might be in it."
It could be any other morning in their lives. Sam puts down his coffee while he pulls on something long-sleeved. They go out.
The talisman is. It's routine; they ask for a cup of coffee and cleanse the thing before the girl comes back with a frown and the actual antique traders, and they get kicked out.
Job done. Another checkmark and a few hundred miles behind them. Another hotel. Sam showers while Dean goes out for food. When he comes back, Sam's clean and in bed and watching the late news. Dean tosses a bagged burger onto the bed beside him, and Sam says, "Thanks, man."
Dean rattles around the room, shedding coat and boots. He crosses in front of the television and Sam just blinks. Eats his burger. A pause in the bathroom doorway, and he says, "Sam?"
Sam keeps his eyes on the television. "This kid's gone missing up near Ansley," he says, mouth full of lettuce and sauce. "Reckon might be worth reading up on at least."
Dean stands a moment longer, but Sam's not looking up. Eventually, he flicks on the light and closes the door.
Sam finishes his burger. Turns off the TV. Lies down.
He's only half pretending to sleep when Dean come out of the bathroom. He rolls away from the light, bleary and stretching his arms out in front of him. Sinking towards sleep and hoping - really hoping - he doesn't have the dream tonight.
He hears, as though through a veil, Dean moving around the room, coming up between the beds. There's the slither-whip of his belt coming out of its loops, and the brush of denim against Sam's fingers over the edge of the bed.
Then there's the clink of metal against the headboard, and just as Sam might've thought of waking up, his wrists are cinched tight together, yanked against the bedpost cold on the back of his hand.
His eyes jerk open, and Dean's just tugging the belt-knot tight. He stalks away, down the bed, out of Sam's vision. Sam yanks at his hands, but the knot's simple and efficient, and his efforts only see it catch a little tighter.
The bed sinks behind him, and Sam doesn't turn - can't turn, his shoulders effectively pinioned, but doesn't turn his head either - as Dean slides across the bed behind him, pushing the covers away. Dean's still wearing his jeans, denim rough against the back of Sam's thighs and the button cold against the small of his back, but his chest presses skin-to-skin against Sam's back.
Sam actually twitches at the hand on his side, realises that his breath is faster when he feels fingers slide over his ribs. Dean breathes measured and steady against Sam's stretched shoulder as his hand moves. Just that one caress, then he pushes down, smears his palm across Sam's stomach, reaching into his boxers.
In two strokes, Sam is hard. He swallows his gasp, but can't stifle the move he makes back, against Dean. His boxers hiss on denim, and then Dean's shoving him forward, into his fist and the grunt just sits on Sam's tongue, his teeth shut against it. Fingers in his hair yank his head back, and his mouth opens with, "ah". His wrists jerk against against the belt, making the bedframe rattle.
Sam could break this, he could get his hands out of this. It would only take a little effort.
He doesn't know what he'd do with them, if he had them free.
His world contracts to this: Dean's fist in his hair and the unwilling arch of his neck, of his shoulders; Dean's fist around his cock, and the grim, implacable rhythm with which he's ripping Sam undone; Dean against his back, hard through his jeans.
Sam comes quickly, and hard, and as silently as he can manage, a breathy moan in hitching triplicate.
Dean lets him go, wiping his palm against Sam's stomach. "Tit for tat," he mutters, sliding off the bed.
It takes Sam barely two minutes to get his breath back under control and his hands free. By that time, Dean's sprawled out in his own bed, on his stomach, to all intents and purposes dead to the world. Sam rolls the belt up and drops it on top of Dean's discarded jeans.
He doesn't mention it. Dean doesn't mention it. They go over the local papers and ask a few questions, and end up going up to Ansley. There's something that looks like a trail of young girls disappearing across the state, and Sam's marking it out on a map while Dean's finding the page in Dad's journal. He drops it on top of Sam's map, taps it with a finger, and says, "Red riding hood," and Sam just nods, brushes it aside, and draws the line.
They know where it's going, so they go there too. The car's as silent as it can be with everything-louder-than-everything-else music blaring.
It feels like fighting, when they were kids. And Sam might win at stubbornness, but Dean's more tenacious, especially when it comes to not backing down. Sam knows he has to do something, but every time he opens his mouth, the silence crawls in.
They catch the thing the next day; Dean calls it the Big Bad Wolf, even though it looks like a frail slip of a middle-aged man. When they have it cornered it shows teeth. Pinning it down is harder than they expected, and even though Dean knocks it out, it wakes up as they're carving its heart out.
They both end up exhausted, tacky with blood and sweat, smeared with ash. Sam collapses next to Dean without thinking, nudges his shoulder. "You alright, man?"
Dean glances over, twitches his foot. "Yeah."
Then the silence is back.
They already had the hotel room, thank Christ, because no one'd let 'em in looking like this, even with a rough-and-ready wash under a garden hose. Sam collapses gingerly on the edge of his bed, head lolling, as Dean empties his pockets in a clatter of coins on the table. "First shower?" he says, and Sam says, "Heads," without looking up, then waves a hand. "You have it, man."
Dean goes. He slings his jacket over a chair, kicks his boots thud-thud into the corner, and goes into the bathroom. Sam takes his shoes off, and smears his thumb over the smears already on the leg of his jeans. Thinks they're going to have to take tomorrow off to do laundry, which is fine with him, because he needs to take tomorrow off to recover from this anyway.
He stands up with a sigh, feeling a good twenty years older than he actually is, and glances over on reflex to check that Dean's ballistic boots haven't left too noticeable a mark on the wall. The water kicks on in the bathroom, with the clink of shower curtain on its rail, and that's when Sam realises.
The noise is too loud, too close. He glances over his shoulder for unnecessary confirmation: Dean's left the door open.
It's not that he's left it ajar, it's all the way open. Two steps around the bed from where Sam was sitting, he can see into the bathroom. Sink, mirror, shower stall. There's steam, because Dean likes his showers hot. The curtain's mostly white, and not nearly opaque enough. Sam can see the shape of Dean, scrubbing his fingers at his hair, ducking his head under the spray.
Sam turns away, and takes off his outer shirt. Pulls his t-shirt up and over his head. Dumps his jeans on top of the whole pile. He's just wearing his boxers (and even they haven't escaped; a spot of blood, ash along the waistband) when he walks into the bathroom.
He goes up to the sink, wipes his hand across the fogged-up mirror. He looks about as ghastly as he feels. Good to know.
Then he turns to the shower, and takes a breath. "Dean."
The curtain twitches back. Not a little, not a sliver, but halfway along its rail. Dean's standing here, wet and naked. "Yeah?"
Sam has this absurd desire to grin. He hooks his thumbs under the waistband of his boxers. "Make the water a decent temperature."
As he's stepping out of them, Dean's saying, "Live with it, bitch," and he's edging back, leaving room for Sam to step into the stall.
Which he does, and tweaks the hot water as Dean twitches the curtain closed again and then Dean's got a hand on Sam's shoulder and Sam's hand's round back of Dean's neck and they're kissing. Sam has to duck his head, which reminds him of kissing girls, but Dean kisses like he's throwing insults, in short, sharp bursts. It's got Sam pushing back in ten seconds flat, biting Dean's lip and laughing when he growls. They were both of them hard before Sam stepped in, and their cocks jostle together as Dean turns them around, pushes Sam back against the tiles - which makes him yelp, because they're colder than the water, colder than Dean's skin.
Sam braces his legs, and Dean steps between them, pushing up against Sam from cock to tongue. Sam fumbles over Dean's water-slippery skin, and gets a hand between them, wrapping his fingers around both of them an instant before Dean's hand is there too.
"Fuck," Sam mutters against Dean's tilted neck, his other hand bracing against the wall, and Dean's head against the tiles by Sam's ear. His breath hitching, and Sam's with it.