The first time they'd had sex (and a handful of times after that) Cas had bottomed. Though the Peeping Tom, as Dean had jokingly referred to him on several occasions, had spent millenia watching humans do everything and anything, from taking a shit to fucking each other in every single way they were physically capable of, he'd never actually done it. And so, as his boyfriend, Dean had figured it was his responsibility to show him the ropes, figuratively speaking.
Once he'd persuaded Cas that he wouldn't break him, however, that changed. Now Cas was the one showing him the ropes, not so figuratively speaking, as Dean had discovered that he actually liked giving up control in the bedroom - to Cas, at least. It had surprised him at first (which had resulted in him freaking out and refusing to have sex for a fortnight and leaving Cas wondering where he'd gone wrong when they were both having orgasms and wasn't that the point, Dean?) but the more he thought about it the more it made sense. All his life, Dean had carried the burden of responsibility on his shoulders.
"Look after your brother."
"Take care of Sam."
"I'm counting on you, Dean."
He'd practically raised his little brother, and then when Sam had left and John had thrown himself even harder into tracking down the demon that had killed his wife, Dean took it upon himself to look after his father. Now their father was dead and he was looking out for Sam again With Cas, though, he could just let go. He could be the one getting looked after. He trusted Cas - the angel had seen him at his worst, and still came back; still vowed to watch over him. Cas wasn't blood - he didn't have to worry about letting him down; about disappointing him.
Cas had told him once that so long as Dean did what he thought was right, Castiel would always respect him, stand by him, and be proud of him. And even if he failed, Castiel would always love him. And, safe in that knowledge, he could let Cas take control with his safe word tucked away in the depths of his throat, like the secrets he'd take to the grave.