Castiel is still awfully new to this whole "human" thing. He's not sure how all the parts work yet. Dean and Sam have been very helpful, reminding him to shower and eat regularly. He's figured out the feelings for when he's tired and thirsty. But this sensation is new to him.
He first felt it when Dean was bent over trying to retrieve something from under the bed, his jean-clad ass in the air for Castiel to gaze upon. He felt a strange, almost painful pressure in the base of his cock. He looked down at the offending area, eyebrows furrowed. Dean straightened up, having gotten the gun that fell under the bed, and stared at Cas staring at his crotch. "Cas, what the hell are you doing?"
Castiel looked up at Dean. He felt that maybe this was the type of thing that ought not to be shared among friends. Still, he wasn't sure how to respond to Dean's question. "I was merely...thinking."
Dean continues to stare at him for a bit before a smile tugs at his lips. "Man, you are one weird dude," he mutters fondly, clapping him on the shoulder as he walks past. Castiel lets out a sigh of relief at the narrowly avoided awkward situation. He looks back down at his crotch, wondering what that stirring meant. He decides to put it out of his mind for the time being and try to figure it out later.
He again feels it when he catches a glimpse of Dean's stomach as he stretches. He could see the V of his hips, and the sight left his lower regions feeling pleasurably tense. He averted his eyes, vowing to do some research later.
When Sam and Dean are asleep, Castiel quietly gets out of bed and boots up Sam's laptop. He goes on the internet, opening up Google to search for a solution to his problem. He types "Why does my penis feel odd when I look at my best friend, Mr. Google Operator?" His results are incredibly confusing, and not at all what he was looking for. He removes the ending of his question, hoping that will eliminate some of the seemingly random results. This does help. From what he can tell, it has something to do with sex. Castiel stumbles onto the term "masturbation." He's heard of it and knows what it is, but not how it is done. He quickly searches the word. Wikipedia has a helpful article where he discovers the technique. He reads up on how to use his hand on his penis, as well as anal masturbation. He's eager to try both.
From what he's gathered, masturbation is something that should be done in private. So he heads to the bathroom, trying not to wake the others. He faces the mirror, finding that he's rather curious about how this process works. The article said he could think of fantasies or past arousing experiences. Considering he's still a virgin, he chooses to think up a fantasy. The first thing that pops into his head is Dean. His bright green eyes, crooked smile, spattering of freckles. The hard lines of his body, the curve of his ass in his jeans. His strong arms, muscular legs. Castiel can already feel that sensation growing in his pants.
He removes his pants and underwear, and then his shirt for good measure. He wants to be able to watch himself properly, so that he can see what he's doing and take note of what's good and what's not. He can only see part of himself in the mirror though, since it is hanging over the sink. Without thinking much of it, he climbs up onto the sink, knees on either side of the faucet. He watches himself in the mirror as he slips a fist around his cock.
Dean wakes with a start from a dream he can't remember. His heart is racing and his skin feels sticky with cold sweat from whatever nightmare his subconscious had thrown at him. He sighs and rubs a hand over his face, calming himself with the knowledge that he's awake and, at least for the moment, safe.
He rises, stretching and yawning, and heads to the bathroom for a quick piss before returning to bed. It's not until he's reached the door and begun to open it that he realizes someone's in there. He peers through the crack in the doorway. He stifles a shocked outburst at the sight.
Castiel is straddling the sink, masturbating furiously at his reflection. His eyes are half shut, mouth hanging open and sucking in shuddery breaths. He's pumping his sizeable cock furiously between his fist, back arching forward with his hips thrusting toward the mirror, eyes slipping shut and opening again slowly.
Dean stares in amazement, cock suddenly and irreparably hard in his boxers. He feels paralyzed by the sight. Castiel has never looked so beautiful, lost in ecstasy as he is at this moment. Dean's hand trails down to his erection without even thinking. He begins pumping in rhythm with Cas.
Dean easily imagines being in there with Cas. Dean perched on the sink, Cas straddling his hips, grinding his erection on Dean's own. Blue eyes nearly consumed by dark pupils, looking down at him with a hunger that Castiel has never shown before. Hands on Dean's shoulders to remain steady, licking and kissing and sucking at Dean's neck, shoulders, jaw. Finally lips meeting in a messy and passionate kiss, Dean's tongue looping around Cas's, moaning into his mouth as he holds onto his slender hips.
What the hell are you doing? his brain suddenly screams at him, jarring him from his enjoyment and lust. He immediately is filled with shame. How can he get off on watching Cas when Cas doesn't know he's there? What he's doing is perverted, wrong, sinful. But even that knowledge isn't enough to make his legs move. Part of his brain is screaming desperately at the complete and utter wrongness of the situation, but his body does not care.
Any and all thinking Dean may have been attempting to do is obliterated when Cas sticks a lubricated finger inside himself. The gasp is followed by a moan and Dean's cock twitches in his grasp. Cas is biting on his lower lip, eyes squeezed shut, eventually relaxing as he finds a steady rhythm. Dean shudders at the scene, too beautiful and gloriously sinful to be human. He continues to stroke himself, pace quickening.
As Cas sticks another finger inside himself, Dean swears he hears the start of a word. "D-" Cas hisses through his teeth. Dean is curious. He pumps himself harder, listening intently for another word.
Castiel appears to be very near orgasm, and Dean is not long to follow. Castiel writhes, thrusting down on his own fingers. He gives a few more fast pumps on his cock, gasping out "Dean!" as he finally cums, splattering the sink, mirror, and his stomach with the stuff. He pants, sliding his fingers out of himself. His release is enough to make Dean cum as well, hands now sticky and wet. Dean watches Castiel relax, muscles loosening in his shoulders. His whole body is trembling from that orgasm-presumably his first. His trembling intensifies, and before he knows what's happening, gravity is pulling him backwards. He cannot reach out and grab onto anything as he falls.
Dean reacts without a thought, calling out "Cas!" as he catches hold of him, arms encircling his bare waist. Castiel is shocked at the sudden intrusion, not to mention embarrassed. He twists around to see that Dean is truly there, grabbing onto his shoulders and squeezing. Dean looks up at him, green eyes ablaze with something Castiel has never seen before.
"Dean...I-" before Castiel can finish, Dean has captured his mouth in a lustful kiss, pulling him full into his arms and holding tight to his hips. Castiel reciprocates a moment later, fingers working their way into Dean's hair, feeling each soft strand as he claws his way through them.
Cas breaks away for air first, new to this whole kissing thing. He pants and looks up at Dean, blue eyes sparkling. "W...what was that for?"
Dean grins down at him, cocky and confident and aglow with joy since Cas seems to have reciprocated his feelings. "Consider it your next lesson on how to be human."
"Will there be any follow up lessons?" Cas asks, and Dean really cannot tell if he's being serious or if he's fucking with him. He grins in spite of himself.
"Of course. There's blow jobs and anal 101 to look forward to." Cas frowns and tilts his head slightly in that familiar confused expression.
"What does 'one oh one' mean?" he asks. Dean laughs jubilantly, leaning his forehead against Cas's and kissing him briefly once more.
"Nothing Cas, it's not important. We should probably clean this mess up though, before Sammy wakes up and throws a bitch fit about all the cum everywhere." Castiel agrees and allows Dean to help in cleaning the cum off of his stomach, blushing lightly at the gentle touch. Dean looks up at him, smiling softly at the red glow to Castiel's cheeks. He kisses one, causing the blush to darken, and turns away politely as Cas pulls his clothes back on, focusing instead on cleaning the cum off the mirror.
When they have erased all evidence of what transpired in the bathroom, the two of them exit. Dean falls in a heap on his bed, smiling in satisfaction. "Umm..." He turns to see Castiel standing at the foot of his bed, nervously playing with a button on his shirt collar. "Could I...umm..." Dean doesn't need him to finish. He scoots to one side and lifts the blanket up, inviting Cas in with him. Cas smiles wide, climbing in the bed and scooting close to Dean. He curls up against his chest, head tucked under his chin, sighing contentedly. "Goodnight, Dean."
Dean smiles and kisses his wild, feathery head. "Goodnight, Cas." They are both asleep in a matter of minutes, comforted by the sound of the other's heart beat.