"This is too... Whoa, Sam, Jesus... Calm down, Sammy, please, it h-"
"What?" Sam deadpans, driving into Dean somehow harder, his hips thrusting in a perfect rhythm. "Does it hurt, big brother?"
"Y-yeah," Dean grits, squeezing his eyes shut. "Please, baby boy..."
Sam laughs now, throwing his head back and shaking it at Dean when he looks back down. "'Baby boy?' Pullin' out all the stops, huh? I'm not possessed, Dean. Your sweet little Sammy isn't gonna hear you and break through the veil somehow. I. Don't. Have. A. Soul." He punctuates each of the last five words with a slam into Dean's prostate.
Dean's gritting his teeth, tears almost slipping from the corners of his eyes.
"You agreed to do this, you know. Like you thought it'd be the same, somehow. Like you thought you could fix me. But look at this, huh? No matter how hard I fuck you, doesn't seem to be knockin' my humanity outta the cage." Sam's voice is even. Doesn't waver. Doesn't break. Almost as though plowing into Dean is having no affect on him whatsoever.
Dean doesn't say anything. Just waits for it to be over.
Finally, after what feels like hours, it is.
Sam pulls out as soon as he comes, stepping back into his jeans, not even bothering with underwear. "It's been real, big bro. Let's do it again sometime." With that, he's out the door.
Dean can't help it now. He just lets the tears fall.