When Dean decides he wants to marry Cas, it's like with everything else. No number of hours were spent thinking it over, it just pops into his head.
It's a completely normal morning in the bunker. Sam had just come back after his morning run and collapsed into the chair, Kevin was already digging into the pancakes Dean was serving and Cas was curled into the chair, knees pulled up and pressed between the table and his chest.
His hands were wrapped around his first mug of coffee and he was glaring at it like it had mortally offended him. His hair was styled messily in a way only bed and sex can - and in this case, it was both - and he was scowling darkly in ways that said that Cas was definitely not a morning person.
But there was nothing that could turn Dean on more than Castiel in the morning, because he was generally annoying and demanding and clingy. His lips were more insistent, his body pressed closer to avoid being left alone in the cold bed and his voice got deliciously low.
That was what made Dean realise he wanted to wake up to Cas every morning for the rest of his life. And the minute the thought formed in his mind, he ignored the breakfast and turned fully to face the former angel.
"Cas" he called without even thinking about it.
"Mmmm" was the non-committal response, in a tone that clearly said "go away”
"Cas" Dean insisted, moving across the kitchen and closer to his lover.
"What Dean?" Cas questioned irritably, finally looking up towards him.
"Marry me" Dean stated.
Stated, mind you. Not asked. It was half an order really, sounding like a mix; caught somewhere between an announcement that said “We’re getting married” and a question of “Will you marry me?”
Kevin choked on his food, Sam flapped a large arm that nearly knocked the unused jar of jam, and Cas, he just looked at him with a “Seriously?” written all over his face.
And if you assumed it was a "Seriously Dean? this is how you propose to me" look then you are wrong. It was a “Seriously Dean? And this couldn’t have waited till I’ve had my three mugs of coffee?” look.
Dean didn’t budge though and kept his gaze fixed on his lover, waiting for a response.
"Alright" Cas half grumbled, before breaking the stare and returning to glaring at his caffeine supply.
Later, Dean would get a ring and go down on his knees and Cas would answer with a more traditional yes followed by a fervent kiss.
Then they would celebrate with so much beer that Kevin would spend the next morning throwing up because try as he may, he couldn’t hold their levels of alcohol.
Later still, Sam would crow with delight and launch into a I - always - knew - you - both - were - meant - to - be - and - I - am - so - glad - you - finally - got - your - head - out - of - your - ass speech that Dean would ask him to can until it was time for the best man to talk at the wedding.
A little after that, Charlie would squeal into the phone and promise that she was getting there immediately and since no one was going to attempt fighting Sam for the spot, Charlie would have to challenge Kevin to a dozen different video games before simply pestering Cas into letting her be the other best man.
All of that would be followed by Dean and Cas lying covered in sweat and breathing heavily under a tangle of sheets, where Cas would decide he wanted to go to the grand canyon once they were married.
But that was all for later. For now, Cas glared at his coffee and Dean rescued the breakfast from turning black, both of them entirely disregarding the fact that they got engaged seconds ago.