Please have mercy, this is my first smut ever.
Castiel was on his knees, his right hand clutching hard at Dean's hipbone and his left at his own side, waiting for instructions. He was slightly uncomfortable, scared of not being able to do this good enough, not being able to give Dean enough pleasure. But for now, he just needed orders.
"Stroke it with your hand first." Dean ordered, his tone gentle as he didn't want to push Cas too much. The angel complied, lifting his free hand and taking a hold of the half hardened cock in front of his face. His touch was light and hesitant and had Dean moaning quietly as he felt Castiel's cold long fingers against his heated flesh.
"A bit stronger, Cas." he breathed out and the angel immediately closed his hand and pumped harder. Dean groaned and ran his fingers through his hair, a small anticipation of what was coming next. "Good, you're good." he praised, knowing that Castiel needed it, needed some encouragement. He was right: the angel's tension on his shoulder eased a little and his hand's movements became more fluid.
"Now...lick it, roll your tongue around the head." Castiel hesitantly opened his mouth and darted his tongue out, letting it flatten right under the head before rolling it over. Dean moaned more loudly this time, his fingers closing through his hair. The movement continued for some seconds and Dean was beginning to trust slowly into his mouth, making him pull back a little as he didn't know what to do next.
"C-close your lips...around it." it came out more as a demand than an order but the effect was the same. Castiel closed his lips around the head, maybe too much because Dean could feel the light edge of the teeth against his flesh. "Cas..." he breathed, "R-relax, Cas...don't close too much." the angel adjusted his lips, pulling back his teeth enough to avoid scraping them on Dean and inclined his head for a better access.
"Now...now suck...go back and forth." Castied obeyed as he bobbed his head, letting his tongue run under the head every time he pulled back. He was loud, slurping sounds quickly filled the room but Dean found it arousing as fuck as he began to pant and thrust forward, slowly losing it by having his dick so far into his angel's mouth.
It was good for a first time and Dean was soon groaning and hissing, his fingers clutched at Castiel's hair as he guided his movements, "Cas...C-cas...fuck...so good...y-you're so fucking good...oh god." he panted as he thrust forward and buried his length down his throat. Castiel's eyes shot up to look at Dean, waiting for an instruction once again. It took Dean a while to notice and he felt a bit guilty for having left him like that.
"I-it's okay...Cas...go on...just like this. It's so fucking perfect...so good...you're so hot." he muttered and he was losing control on his brain, the words leaving his mouth as soon as they were processed. He was close, focused on Castiel's tongue swirling around his head when he pulled back, on his tight throat when he pushed forward and on his slurping noises when he was halfway. The angel soon began to push forward, meeting Dean's thrusts halfway and deep-throating him every time, slightly gagging but able to breathe when Dean pulled back. He loved to hear Dean's moans and groans, his praises and appreciation, they all made him feel pleased and satisfied.
"Cas don't stop...don't...uh...oh god...I'm going to come...Cas." he choked out and started to pull out but as he tried Cas tightened his lips and his teeth lightly scraped him, making him come right there and then inside his mouth with a loud yelp. Dean's face grew hot and red in shame as he watched Cas looking up at him with his mouth full. He didn't really mean to do that, not during his first time at least.
"I'm...so sorry, Cas." he apologized as he tried to regain breath. "You can...you know...spit it in the sink if you don't want to...uh, swallow it." Castiel seemed to consider the idea, but in the end he swallowed all of it as he decided that going to the bathroom right then and spit all of it might have ruined the moment.
Dean watched him as he did and he couldn't deny that the simple sight of that was slightly arousing and he soon joined Castiel on the floor, kissing him hungrily as he tasted himself in his mouth. The angel's erection laid untouched inside his black trousers and the hunter ran his fingers on it, making Castiel shudder at the sudden intense sensation, "Now let's take care of you, huh?".