A Tumblr prompt: Laundry Mix-Up. Sam accidentally puts on a pair of Dean's underwear. Things turn naughty... and then flat-out kinky.

Sam tugged at his underwear. "Dude, did you like wash our clothes in super-hot water or something?"

Dean looked puzzled. "I do it like I always do."

"Warm wash, cold rinse?"

Dean rolled his eyes. "I know how to do laundry, Sam."

"Then why is my underwear crawling up my ass?"

Dean started laughing.

"What?"

"Dude. You're totally wearing my underwear."

Sam blinked. "Fuck.

"You can't get enough of me, and you grabbed MY underwear instead of yours because you wanted something that's usually wrapped around my junk all tight up against yours." Dean grinned like a little kid on his birthday.

Sam tipped his head, embarrassed. "Cut it out."

"Sammy, you dog. Kinkier by the day. And I love it."

Blushing harder, Sam retorted, "I did NOT do it on purpose. And I don't have a… an… underwear fetish."

"C'mon. Admit it. It gets you hot." Dean was really enjoying this. But he always loved teasing Sam.

Sam shifted in his seat, trying to keep the motion subtle, but Dean noticed. Saw the tell-tale shape beneath Sam's jeans.

Dean sucked in a breath. "Christ. It does get you hot."

Sam said nothing, but his breath came slightly faster.

Dean pulled the car to the side of the road, coming to a stop behind a line of trees shielding them from view from the road.

"Show me."

Sam gnawed on his lower lip and unbuttoned his jeans. His cock was fully erect, the head peeking over the top of the too-small boxer briefs.

Dean made a little sound low in his throat. "Yep. Those are mine."

Sam ran his hand through his hair. "I know that now."

"Feel good, Sammy? Wearing my underwear?" Dean traced a fingertip along Sam's cock, over the cotton fabric.

Sam shivered.

"Pull your jeans down." Sam shucked the jeans down to his knees. Dean pressed his hand against his not-so-little brother's erection, drawing a low moan out of him. "I like that. Like seeing you wearing them." Dean gripped Sam's cock through the underwear and began jacking him off.

Sam threw his head back and gave himself over to Dean, the way he always did, the way Dean loved. Sam didn't give it up like this for anybody but Dean. Dean never had been with anyone so pure and debauched at the same time. It was astonishing—and addictive.

Dean pulled his hand away "Wanna see you come for me, Sammy." He straddled Sam, still fully clothed, and rubbed his denim-covered cock against Sam's, pumping against him, sealing his mouth over Sam's so he could swallow all his delicious little cries and gasps.

He didn't tease Sam, but writhed against him like a pro, trying to get him off hard and fast. "Come for me, Sam." And Sam could never say no to Dean. He came for Dean, clutching his back, hips bucking uncontrollably, soaking through Dean's underwear, breaking the kiss to cry out only to have Dean grab his face with both hands and force him back, took his mouth again, made Sam groan and half-scream his orgasm into Dean's mouth.

And that was all Dean needed. He came shuddering, chanting Sam's name, grabbing onto his shoulders, his hair, his arms, trying to keep himself from flying apart into pieces.

They panted against each other, catching their breath, letting the sweat cool. Then Dean climbed off Sam. "Jeans. Off." Dean pulled off his boots so he could remove his own jeans.

"What are you-?"

"Sam. Jeans. Off. Now." Sam shook his head and pulled his trainers off, then took off his jeans.

"Underwear." Sam peeled the underwear off, come soaked through the front.

Dean did the same thing.

Then he held his come-soaked underwear out to Sam.

Sam stared. Then his eyes widened.

"Yeah, Sammy. You're gonna wear these. All day. Feel my come drying on your cock." Sam's cock twitched visibly, trying to get hard again.

Dean's smile was full of wicked promise. "Knew you'd like that, baby boy."

Sam licked his lips. "And you?"

"Oh yeah. I'm gonna wear yours. Wanna feel your come on me too." Dean pulled on the boxer briefs Sam had been wearing, closing his eyes and groaning when the come-soaked fabric slid along his cock. "And later?" Dean leaned forward and kissed Sam, so slow and sweet it made Sam dizzy. "Later, I'm gonna make you lick it all off me. Clean me up real good."

Sam shuddered. "Christ, Dean. You're gonna fucking be the death of me." He took the underwear from Dean's hand and put it on, biting his lip when the wet patch slipped over his cock, already half-hard.

Dean smirked. "Hey, dude. You're the one who mixed up the laundry."