Sam reasoned it was just one of those things. Like when you're surfing for porn, and you're already nicely on edge, and it won't take much to get you there. And then you just happen to trip over something which you'd normally never look at, because ew, but you're close and somehow that one nasty thing just does it for you. But the instant the haze of orgasm fades, you shut down that thing faster than the speed of light. Because you might just have gotten off spectacularly, but that doesn't mean you like that sort of thing.
It was just like one of those things. Sam kept telling himself that as he double checked that the bathroom door was locked properly, before stepping into the shower fully clothed. He probably wouldn't get another chance to do this anytime soon, so now was the time. It was just an experiment. It wasn't like he was into it a lot or anything.
He could hear the TV on the other side of the wall. Dean had taken first dibs on the shower, so he was probably splayed out on his bed with a beer by now, leaving Sam to deal with his own mess. Whatever that thing was they just killed, it had sprayed slime all over them. Dean's jacket had gotten a broad path of goo right across the back, and Sam had gotten various spatters from head to toe. Dean had been sympathetic for once, and only taken a quick shower, so Sam would have enough hot water to deal with his clothes and his hair.
As he stood in the stall, staring stupidly at the shut off shower head, he wondered yet again if he should really do this. But he was with Dean 24/7. If he was gonna try this, now was his chance to do so while avoiding awkward questions. Casting yet another nervous glance at the door, Sam spread his legs slightly and relaxed as much as he could. It took an effort, since he was apparently hardwired not to do this, but finally he managed to let go.
A small stream of piss made its way down his thigh, and he gasped sharply before slapping a hand over his mouth. Supporting himself with his other hand on the wall in front of him, Sam pushed a little and felt the hot liquid spread out across the fabric of his dirty jeans. It was warm, so amazingly warm, but it cooled fast at the edges, and sent conflicting messages to his cock. He was getting hard fast, so he pushed more, hoping to finish while he still could. His cock chafed against his wet boxers, but somehow that was just right, and he reached down to palm himself roughly, still covering his mouth. Which was a good thing, because the pressure on his still trickling cock was overwhelming, and he rubbed himself furiously through the fabric, small moans escaping, only barely muffled.
He was fully hard now, and his cock was bending slightly inside his jeans. It was almost too much, but the slight twinge of pain combined with the rapidly cooling wetness was such an amazing feeling that it only fueled his lust. He forced out one last spurt of urine, feeling the warmth in his palm, rubbing against his cock head through the clothes, and he shivered. The first wet patch on his thigh had now cooled entirely, and he squirmed in his pants, unsure whether he wanted to get closer to it or away from it.
His hand shook as he slid it up to his stomach and then back down to awkwardly worm its way into the damp jeans. The sodden denim against the back of his hand made him whimper, and when he finally got hold of his cock it felt burning hot in comparison. He jerked himself desperately as much as he could in the confined space, and his thighs shook as he came, his come adding new warmth to the cold spread of urine reaching from hip to knee.
He finally took his hand off his mouth, using it instead to brace himself against the wall again. He panted harshly, his other hand still trapped in his wet pants, his knees wobbling under him. As he slowly resurfaced from his bliss, it was distressing how cold, filthy and ashamed he felt. He turned on the spray, soaking all his clothes thoroughly with hot water, as if boiling them would make it so that it had never happened. Because it wasn't like a fetish for him or anything. It wasn't.
But the hot water trickling in through his collar before making it through the layers of fabric only made him wonder what it would feel like if the urine had been running down his chest instead of seeping into his pants.
In the motel room Dean chuckled at something on TV, while Sam struggled to scrub away the latest evidence of how damaged he was.