Set during the night of "Abandon all Hope" (Season 5 Episode 10). Bit of Despairing!Dean, and a lot of Innocent!Castiel. And sex too. Sex. Yep.

Dean sighed into the pillow. "If you want to have sex, you should just say so."
Castiel blinked. "I was not aware you were awake."
Dean shifted and looked at the angel standing silently in the corner of the room.
"Trust me, in the Hunter business you learn to notice when people are watching you sleep."
Castiel tilted his head, and apparently accepted that as a reasonable answer.
Dean sat up, resting his arm on his knee casually. "So, this is the part where you take your clothes off."
Castiel frowned. "Dean, I…"
Dean sighed. "Cas listen. We're going to die tomorrow. And as I said earlier, there's no way I'm going to let you die a virgin."
"There's a chance-"
Dean growled. "I don't want to hear it. Not that crap. Not now. Take off the trenchcoat and come here."
Cas' frown deepened but he obeyed, letting the trenchcoat drown unceremoniously into a pile, and sat on the edge of Dean's bed.
Dean relaxed slightly, pulling himself out from under the covers to sit next to Cas. The angel was looking nervously away at the bedside table, which brought forth a chuckle from Dean.
"You look like you did in the brothel."
Cas' eyes met Dean's. "Dean-"
But Dean didn't want reassuring statements. Instead he reached over and pulled the angel's lips onto his lips and kissed him passionately.
Dean chuckled again as Cas' eyes widened like saucers. Cas' gaze then fell to his lap, where his erection was quite visible through the fabric. Then his eyes narrowed and Cas leant in, surprising Dean with a kiss of his own.
The surprise only lasted for a moment though, and soon Dean was leading Cas to lie back on the bed as he manipulated the shirt off Cas' back. He leant in and kissed Cas passionately, then directed his mouth to Cas' ear and whispered harshly, "Cas, if you need me to stop, or if you have questions or whatever…"
Cas nodded and squirmed, startling yet another chuckle from Dean. If he'd known how fun this would be, he thought as he trailed kisses down Cas's bare chest, he would've jumped Cas quite a while back.
Suddenly Cas stiffened, and Dean lifted his head, concerned. "Cas?" He panicked suddenly, wondering if he'd gone too far, too soon.
"Dean, I'm leaking." Cas said, a tone of nervousness evident as he angled his head to try to figure out the problem.
Dean gave a shaky laugh and rested his head briefly on Cas' torso. "That's normal, Cas. It's called pre-cum."
"Oh." Cas replied and let his head fall back onto the bed. There was a pause as Dean collected himself. "Please continue." Cas said finally.
Dean looked up at him, to find the blue eyes staring curiously at him. Dean frowned and then rolled his eyes, sitting up and beginning to pull at the belt. As he did so, he accidently grazed Cas' erection and produced the first moan from Cas. Dean froze again, as his body filled with lust and longing. Cas didn't have to beg for Dean to continue this time, Dean suddenly surged with adrenaline, ripping off the belt and pulling down the pants. He then calmed himself, sitting near Cas' feet, reminding himself that this could be the last time he had sex, and god dammit he was going to enjoy it, Lucifer be damned. As he admired the view he grabbed Cas' shoes and took them off. Cas' eyes were closed now, which Dean was both annoyed and pleased about – on one hand, he had recently realised that he loved the colour of Cas' eyes, on the other, he was glad that he didn't have to try to figure out what the angel was really feeling anymore. After taking off the shoes and throwing the pants across the room to join the trenchcoat, Dean began another round of kisses, slower this time, starting from Castiel's ankle and working his way up the pale skin to the thighs. Cas' back arched as Dean moved upwards, and he groaned silently. Dean's mind searched for a pun, but other concerns were taking over the 'rational' part of his mind. He kissed the skin between the leg and the hip, enjoying Cas' involuntary movements as he strained. Then Dean moved his mouth, enclosing Cas' cock inside his mouth and began to suck. To hell with stretching this out, he thought to himself. How many times does one get to fuck an angel? His tongue circled the cock expertly. Dean hadn't been exactly the straightest guy over his lifetime, as who knew how long that lifetime would last? Of course, Dean's role was normally that of Cas', but he figured he'd top this time. He'd rather not go into battle with an aching arse. Cas on the other hand… He pulled away his mouth suddenly, to the protests of his lover, which stopped as Dean slid off the bed.
"Dean," Castiel began, opening his eyes and barely suppressing the emotions running through him, "This is cruel. I need-"
"Oh I know what you need," Dean assured him, as he began searching through his bag. "Trust me, I feel you." His own cock was aching with the same barely suppressed need that Cas' was going through.
Cas sat up, shivering slightly. "Dean," he repeated, more desperately.
"Just a sec. I have it here somewhere," he muttered. "Got it," he said as his hand finally gripped the lube he kept, just-in-case, and turned back to Cas. He blinked in surprise to see that the angel had decided to take 'matters' into his own hands. Dean tutted and walked over, slapping away Castiel's hands. Cas made another desperate noise.
"Turn over," Dean began. "No, wait," he said, as Cas obeyed. He looked down. "Hmm," he murmured. He looked up and gave Cas a cheeky grin. "Help me take off these clothes."
Cas blinked and then rose to his knees on the edge of the bed, making short work of Dean's clothes. Now naked, Dean pushed Cas back onto his back. "Now you can turn over," he told the angel, who obeyed as best he was able, on his knees once again. Dean sat behind Cas' awaiting arse and dipped his fingers in the lube, and then letting them circle the entrance. Cas whined. Dean grinned. "You know, I thought you angels had more patience."
"Dean." Cas began through gritted teeth, "I am not in the mood for jokes."
Dean's grin widened, and let one finger slip into the hole. Cas groaned. "Then what are you in the mood for?"
"I believe you already know," Cas said panting, "so I won't bother explaining."
"Not into dirty talk," Dean remarked, slipping in another finger, to another moan. He gave the patented Dean-head-tilt of acceptance. "I can deal with that." He pushed in the third finger.
"Dean." The word was barely formulated now, interjected with shaky breathing and breathless hisses.
Dean took out the fingers, letting them trail across Cas' arse and admiring the trail they left against the skin.
Then he suddenly pulled Cas close, down onto his own dick, meeting with no friction, sliding seamlessly into Cas.
Cas exploded into a wordless yell, which made Dean regret his sudden movement. Hopefully no one heard it, and if they did, he hoped they'd figure out what was going on, otherwise it'd be embarrassing.
"You okay?"
"Mm," Cas said. He sounded okay. "Please now Dean," he added.
Dean chuckled and reached around to grab Cas' cock even as his hips began to rotate. Cas fell forward slightly, grasping at the bedspread. Dean leant forward and kissed the paleness of Cas' back.
"You holding up okay?"
"Yes."
Dean sped up slightly, both hip-wise and hand-wise. Cas was a rather vocal lover, he reflected as Cas mad small noises of pleasure. Dean closed his eyes, letting himself enjoy this moment. This was the first time he'd been with someone he really cared about, he realised suddenly. The realisation was so shocking, he barely noticed Cas' back arch as he came.
Cas panted heavily, sitting up slightly.
"Hey," Dean said (panting as well), "you're not the only one here, you know."
"My apologies." Cas replied.
Dean rolled his eyes, and let go of Cas' cock, shaking his hands to get rid of the majority of the cum, and let his hands pull Cas' torso closer to him as he sped up once more. Since Cas had come, he felt no need to hold back, and rode his own waves of pleasure to completion, and groaned (the first unplanned noise of the session) as he came. He held Cas to him still.
"Thanks." He said.
He could hear Castiel's frown as he replied, "I don't understand."
Dean disengaged himself from Cas and slid off the bed. He waved to the aforementioned bed. "For the uh…sex."
Cas' frown deepened. "Dean, I… I believe I love you."
Dean froze, halfway through picking up his discarded top. He looked Cas over. And he smiled and shrugged. "I guess, since we're in for a showdown tomorrow, there's no point dancing around issues, is there?" he paused as he straightened and put on his shirt. "I love you too Cas."
And part of him delighted to see the beautiful blue eyes of his angel light up with happiness.