Warnings: animal traits (knotting), hints of mpreg

Dean was nervous. He was trying to hide it, of course- but he was nervous about letting Sam fuck him.

It's been three months since Castiel's death, ten weeks since the reports of anomalies started cropping up all over the mainstream media. Six weeks since people stopped freaking out and admitted the truth- that somehow, for some reason, people around the world had started developing new biological features.

Two weeks since Sam jerked himself off in the bathroom and found himself with a knot on his cock.

He wouldn't have told Dean- would have kept it to himself forever, even if it meant making nice with his old friend celibacy again- but Dean barged in when Sam wouldn't answer his calls through the door, too distracted by how he couldn't stop coming, the floor of the shower coated in thick strings of his come, too much for the water to wash away.

Two weeks since Sam turned to face Dean and sprayed him in the face with his endlessly erupting cock and Dean just stood there and took it. Sam's been impressed, even in the throes of orgasm- getting that sort of distance was rare even at the best of times.

Dean had refused to talk about. He'd gone into full-on denial mode, even as he washed his face with shaking hands. He hadn't gotten it all, missed some of Sam's come in his hair, under his ear, but when Sam pointed it out all he got was a "We're not talking about this, Sam. End of story."

It only took a week after that for Sam to walk in on his brother fucking his hand and moaning Sam's name.

Part of Sam- the part that's quiet, most of the time, but flares up in flashes of fire and pain- had wanted to drop it. Drop it, not talk about, huddle down into a little crying ball and denydenydeny. Another part (the toohungry, toohorny, toocold part of him Sam wishes he could claim no relation to) had wanted to flip Dean over and fuck him right then and there, no matter the consequences, no matter the protests. It always felt so good making the bitch's cries turn from protest to pleasure, didn't Sam remember? The tightwetheat of a girl's cuntassmouth, the desperate look in a guy's eyes when Sam hammered on his prostate so hard he didn't need a reach around- that part was harder to ignore, 'cause it pitched a fit if it didn't get its way.

Sam didn't do either of those things. He waited until Dean couldn't get away and they were on a long stretch of highway and a full tank of gas and a ticking clock pushing them ahead, and then he talked. Confessed it all- every fucked up fantasy, every half-stolen glance, every moment Sam spent crushing his feelings into a box and shoving it so deep he hadn't thought to open it even when his soul was taken out of the picture.

When Dean didn't answer, Sam stretched out his hand and let his hand rest on Dean's crotch, where his hard cock was pressing up against the seem of his zipper, shamelessly slutty in a Dean would never be.

After that, it was just a matter of time, and Sam was nothing if not patient.

"You're sure about this, Dean?" Sam asked, palming himself through his boxers.

"'Course I am," Dean said. He didn't move out of the bathroom doorway, eventually reaching out to flip off the bathroom light. He was wearing a tight-fitting pair of boxer briefs, showing off despite his hesitance.

It was late, even for them; three am pushing four. Sam would have offered to let it slide, but waiting until after the hunt had been Dean's final bargaining chip, and Sam wasn't going to give him any more slack on this. Sam wanted it. Dean wanted it. It was happening.

Sam rolled his eyes and folded his hands over his stomach. He stared at Dean.

Predictably, silence worked when cajoling wouldn't- Dean broke away from the door, one halting step melting into a smooth stride to the bed. He stopped at its edge and one deep breath later, was on top of it next to Sam.

"See?" bragged Dean, crawling up the bed and sprawling himself out next to Sam. "No problem."

Sam bit his tongue and rolled over, sliding easily into the hollow of Dean's bow-legs. Before Dean had a chance to quip about- fuck, anything, who knew with Dean- Sam bent forward and captured Dean's lips in a slow, deep kiss.

Dean always was a man of action rather than words. His hand reached up and tangled itself in Sam's hair, and his mouth spread open in a satisfied exhale. Sam took the opportunity to lick inside Dean's mouth and smiled when Dean let out an eager noise at the intrusion. Dean's tongue chased Sam's back into his mouth and Sam used the distraction to slide his hand under the waistband of Dean's boxer briefs.

Dean moaned, long and slow. "So fucking hard," he panted into Sam's mouth, thrusting his cock through Sam's grip. He shuddered and spread his legs further, so Sam took the opportunity to slide his cock so that the head bumped against Dean's perineum and pushed forward and smiled as his cock slid down. He took hold of the cheeks of Dean's ass and spread them and his cock slid between the cheeks of his brother's ass, then let go. Dean's ass cheeks came back together around his cock, creating a tight, sweat-damp channel for him to fuck.

"You gonna be good for me, Dean?" he asked, twisting his wrist on each upstroke. "You gonna let me fuck you with my new knot, let me fill you up?"

Dean arched up and brought his legs around Sam's waist, hooking his ankles and using the support to drag Sam in closer. "Yeah," he breathed, and leaned in to suck at Sam's neck, his teeth pressing in in a soft bite.

Sam groaned and arched his neck to allow Dean better access, trying to angle his hips to catch against the dip of Dean's tight little rim, instinct driving him to fuck, breed, pump Dean up till he was fat with come.

"Shit," Dean muttered and angled his hips upward, so Sam's cock caught on his hole every time he slid his cock forward. "We need lube, fucking, get the lube, Sam."

Sam propped himself up on his arms and dragged himself away from Dean's body, unhooking Dean's legs from around his waist with one reluctant hand. He crawled backward out from the splay of Dean's legs and pressed a kiss to the inside of Dean's ankle as he went. Dean shifted himself up onto his elbows and watched Sam with dark, heavy lidded eyes as Sam dug into his discarded pants for the lube he'd left there earlier.

He paused beside the bed and tossed the tube to Dean, who caught it automatically. He blinked at Sam, who grinned. "I think you should put on a show," Sam said, settling himself on the corner of the bed and turned so he faced his brother. "You get yourself nice and lose for me, yeah?"

Dean flushed red, right from the tips of his ears down to spread over his chest. He ducked his head and stared at Sam through the fan of his lashes; he bit his lip in what seemed to Sam a deliberate tease and squeezed out a dollop of the lube on to his fingers. He shot Sam a heated look and rolled over in a deliberate tease, propped up on his knees with his shoulders dug into the bed covers, his back a startlingly smooth line of golden flesh leading to the smooth swell of his ass. He spread his legs wide and reached between his legs to lightly touch the rim of his hole.

Sam gripped the base of his cock with one hand. "You ever do this?" he asked, voice husky. "Or maybe you let one of those girls you used to like so much fuck you open on their slim little fingers?"

Dean's voice was muffled by the bed. "Never used to," he panted and dipped a finger inside, a quick flick, before he readjusted and hooked the bare tips of two fingers into his hole and pulled downward, so his hole gaped open for a moment. "I've been fucking myself in the bathroom since I walked in on you," he confessed.

Sam sucked in a sharp breath and crawled up the bed and lay himself out next to Dean. He ran his hand down the dip of Dean's spine and then sprawled himself across Dean's back, his larger frame bracketing Dean's. His cock settled back into the cleft of Dean's ass, so Dean's fingers were pressed against the shaft as he fucked himself. "You been thinking about me?" he asked, voice husky. He settled his weight onto his knees, grateful for his long legs, and reached around Dean's chest to grip his brother's nipples. They were already hard, peaked with Dean's arousal, so Sam pinched the tight buds between his thumb and index finger and pinched.

Dean swore and Sam could feel him readjust as he pressed a third finger into himself. "I'm ready," he muttered into the mattress, and pulled his fingers out to grip Sam's cock. "Come on, I'm lose, fuck me."

Sam felt his eyes cross lightly as Dean maneuvered the head of his cock to brush against Dean's loose hole, barely able to brace himself before Dean thrust himself backward, impaling himself on Sam's cock in a quick, jerky motion.

"Shit!" he swore, and Sam couldn't help but agree. Dean's rim was loose, but he hadn't been able to get the depth needed to properly prep him. His ass clamped down on Sam's cock and didn't release, body cramping up.

Sam shook himself and withdrew, the tight heat painful even for him. "Shhhhh," he soothed. He sat back down on his ankles and gave Dean's hole a long look. It was a tight little rosebud, now, all clenched up and resistant despite the previous prep. Dean's legs were trembling and he collapsed to the bed, pained and, Sam noted, still hard. "You're a virgin, Dean," Sam said quickly. He dragged a finger up from Dean's balls to his hole, gathering the small amount of lube that had leaked out. "You need better than just some shallow fucking if you're going to take my knot."

"'m not a virgin," Dean muttered and twisted so he could glare over his shoulder at Sam. "You just have a mutant cock. I'd be able to take you just fine if you weren't a giant."

Sam grinned at his brother, and pressed two fingers into Dean's hole without preamble. Dean jerked as Sam's fingers slotted immediately into his prostate and let out a long, slow moan as Sam rubbed his fingers in a slow circle over it. "Virgin~" Sam sing-songed, and added a third finger. Dean gripped the sheets and fucked himself back onto Sam's fingers. He relaxed slowly, stopped fighting the intrusion of Sam's fingers. "That's it," Sam said, relief pouring through him. He didn't want to hurt his brother, and if Dean thought his cock was big, he obviously hadn't gotten a good look at Sam's knot. Sam scissored his fingers thoughtfully, and added a fourth. In the back of his mind, it occurred to Sam that he should have needed more lube for this, but Dean's hole was slick as a horny girl; his brother must have poured more onto his fingers than Sam had noticed.

Sam slid his fingers deeper into Dean, and his breath caught as the red rim of Dean's hole popped over the line of his knuckles and took Sam's hand in up to his thumb. "Fuck, Sammy," Dean panted into the covers and struggled to spread his legs to make room for Sam. Sam drew his hand back until just the tips of his fingers were curled into the rim of Dean's hole, and pressed outward, so the clenched circle of muscle was pressed outward, toward the surface. Sam ran his thumb around outer edge and felt his own fingers pressing back at him and twisted his hand in a circle as best he could, tracing the entirety of Dean's sphincter. "Such a pretty hole," he breathed, and licked at the muscle his fingers had brought into definition.

"You're a dick," Dean said, voice high. Sam glanced up the line of Dean's body and med Dean's eyes from where they glared at him over Dean's shoulder. "Stop fucking around and fuck me already."

"Pushy," Sam muttered into the skin of Dean's ass, hiding his grin. He raised himself up and gripped one of Dean's thighs and flipped his brother easily onto his back. "You think you can take me now?" he asked casually, and moved his cock down to Dean's hole, circling the loose rim with the head of his cock. He pushed in just far enough that the head popped in and then drew it back out, so Dean's hole had to flare out to release him. "Your hole seems to think it's ready."

"Sam," Dean whined. He groped for Sam and dragged him forward, so they were pressed chest to chest. "Just- fuck me, come on," he captured Sam's lips in a slow, deep kiss and sucked on Sam's tongue until Sam cooperated and started fucking his mouth. Sam waited until Dean arched into into the kiss, tangled his fingers in Sam's hair and then thrust his cock in.

Dean tried to tear his mouth away, but Sam followed him and kept his flicking his tongue out, tasting Dean's mouth and fucking his ass. He kept their lips sealed as he bottomed out, the delicious slickness of Dean's ass spurring him on.

Sam settled in as far as he could go, the barest inch of his cock free from the tight, clenching heat. He ground his hips in a slow circle and pressed his chest down on Dean's back, crushing his brother to the mattress. "There," he breathed into the skin of Dean's neck. "How's that, Dean?" He drew his hips back in a slow movement, and ground forward again. His knot hadn't started to swell yet, but he could feel the blood pooling at the base of his cock, a clear warning.

"It's good," Dean said, voice low. He craned his neck back and caught Sam's lips with his own. Dean's lips were soft, too-tender; Dean'd been chewing on them, pressed down on the mattress where Sam couldn't see. Sam bit down on Dean's lower lip and sucked. He wanted those lips puffed out, as well-fucked as Sam was going to leave Dean's ass.

Sam slipped a finger down between their bodies and curled it along the tight line where his cock was fucking into Dean. He ran the pad of his finger lightly along the stretched muscle and pressed his finger in alongside his cock. "Fuck," Dean whined into Sam's mouth, and Sam could feel it as Dean's body tried to tighten up, muscles clenching twice in rapid succession before falling limp, loose and open to Sam.

"That's good," Sam grunted and snuck another finger into Dean's hole alongside his cock. The smell of sweat and musk was heavy in the air, and it made Sam's head feel heavy, too-full, and spurred him on. He left his fingers in Dean's ass and drew back until Dean's ass was held open on just the head of his cock and the tips of his fingers and then fucked back in, deliberately aiming for Dean's prostate. Dean gave a whole-body twitch and his hole clenched greedily on Sam's cock and fingers, eager as a virgin on the fast track to slut.

Sam withdrew his fingers from Dean's hole and started plowing his brother, long jabs with his cock fucking into Dean's ass. It was a smooth ride, slick as could be, and Sam grunted as he felt the first swellings of his knot.
"Sam!" Dean
cried. He ground his ass back onto Sam's cock, and Sam watched in fascination as his slowly swelling knot pulled at the rim of Dean's hole each time he drew back. Dean's hole gave in easily on each stroke inward, desperate to be filled, and clung around Sam's knot as he withdrew, reluctant to give up its prize. Every time he withdrew Dean's ass made a wet pop as his rim gave way over and over again.

Sam didn't want to fuck Dean's ass sloppy, though. He wanted his come to stay up inside Dean as long as possible, the instinctive need to plant his seed overpowering in its insistence. Dean wasn't fertile, couldn't catch the way Sam wanted- but Sam's body didn't believe that. Sam slammed in and stayed there, just rotating his hips so his cock brushed over Dean's prostate as his knot pumped up to its full size.

"Shiiiit," Dean wheezed out. He was a twitching, sweat soaked mess, his hole glistening with the pungent lube he'd used to open himself up. Sam let himself just grind for a few minutes, testing the tight hold Dean's rim had at the base of his cock, and let his eyes roll into the back of his head as he started to come.

A full body shudder worked its way up from Dean's toes and his ass started spasming around Sam's cock, milking it as Sam kept dumping his load. "Feels so wet," Dean whimpered. He started rocking back onto Sam's cock, the rim of his hole pulling tight as Sam's knot kept him buried deep inside. The pressure just seemed to spur Dean on, and fucked back on Sam and humped the mattress with his cock. Bitch, whispered some dark part of Sam's psyche. Bitch in heat.

If only, Sam thought, even as he started up a slow grind, his balls pulling up as he pumped another load into Dean's ass. "Imagine if I fucked you pregnant like this," Sam grit out. Dean let out a shocked gasp at the words and jammed his hand down under himself to start jerking his cock. "Would you like that, big brother? Get so fucked full of come you swell up with the next generation of Winchesters?" Sam nuzzled Dean's neck and then bit down, instinct driving him to claim, mark, own.

Sam lifted himself off his brother's back and pulled Dean up along with him, cock dragging Dean into a kneeling position. He shifted his weight, spread his legs to anchor himself, and slid his hand around Dean and smacked Dean's hand off his cock. "You like my knot?" he asked, pressing a line of kisses up Dean's neck. "You like being pumped full of more come than your body knows what to do with?"

"Yeah," Dean said. He sucked in a harsh breath as Sam started fisting his cock. "Wish you could fuck me pregnant," he said. "Just fuck me so hard, so good, my body thinks it's a girl and swells up with you. Swell up like your knot." Dean threw his head back, almost knocking into Sam's nose.

Sam moved his hand from Dean's cock to comb through his pubic hair and spread his fingers out over Dean's abdomen. "I can feel my cock, Dean," he whispered into Dean's ear. "Feel my spunk filling you up- can you tell, Dean? Your belly's already starting to round out with my come." He stroked his hands over the almost imperceptible swell of Dean's abdomen, probably more a figment of his imagination than anything, but- "You'd swell up just like this if you were my bitch and I got you pregnant," he said, and Dean came.

"Fuck!" Dean cried, and Sam quickly moved his hand back to Dean's cock and started pumping, drawing out the process. Dean's ass was clenched tight around Sam's knot, and Sam's cock happily pumped a few more loads worth of come in response. Sam's hand was wet with Dean's come, and he spread the come around Dean's cock like lube, until Dean's cock was filthywet mess, rank with the smell of their rut.

Sam waited until Dean calmed down from the orgasm, and then starting pumping his hips again. His cock was still happily coming in Dean's hole, pumping more come than he knew he had in him up tight inside his brother. He was still horny, but more than that, he was lazy- and he wanted to watch Dean. He slid in deep and curled himself around Dean's body and rolled them, so Dean was splayed out on top of him. He fuck up and smiled as his knot slid in further, Dean's ass more open now than ever.

Dean arched on top of him, and he slowly arched himself until he was sitting perched on Sam's cock. He peeked at Sam over his shoulder and his lashes fluttered shut, ass clenching extra-tight on Sam's cock before loosening. From this position Sam could feel the extra wet of his own come in Dean's ass, making every movement easy as fucking a pussy.

"Turn around," Sam
said, eyes intent on Dean. Dean shuddered and nodded, on leg swinging over Sam's, then the other swinging back over Sam's torso, the movement fluid, instinctive- Dean just ground down on Sam's knot as he turned, so there was no chance of his cock slipping out even if it hadn't had a knot keeping them tied.

The sight of Dean perched on his lap, stuck on Sam's cock while his own hung limp and wet, the head just brushing Sam's belly, sent a hot coil of possessiveness through Sam. "You gonna fuck yourself on my dick?" Sam asked, voice inquisitive and far too steady for how much he wanted to watch Dean get off on his cock again.

Dean shifted on top of Sam, testing how far he could pull away from Sam's knot before the tight pull forced him back. Sam grabbed Dean's hands in his own and held them tight. He could feel Dean's pulse beating wildly against his palm and rocked his hips upward to meet the motion of Dean's hips. "You're beautiful," he said.

Dean flushed and gripped Sam's hand tight, head tilted away. Sam widened his stance and braced his feet against the bed and thrust up hard, lifting Dean clean off the bed. "You're going to keep fucking yourself until you get hard again," he promised. "And then I'm going to make you come on just my cock, on the feeling of me do deep up in you, from the feeling of my come filling you up. Yeah?"

"Yeah," Dean agreed, voice breathy. "I think I can do that."