He can't even help grinning widely against Castiel's skin. There's a juxtaposition between Castiel's innocence and his willingness to shed it that Crowley finds incredibly alluring.

He wants to break Castiel of his shyness, but he also wants to treasure it a little. How often does he getting a literal stammering, blushing virgin in his bed, least of all one who's also an angel of the Lord?

"You want my mouth on you?" He presses an open-mouthed kiss to Castiel's jaw. "Where do you want it?" He's going to force Castiel to say it. There's a thrill in having this kind of power over a being like Castiel, especially when he knows what Castiel can do to him. The angel is here of his own volition, because he finds Crowley desirable; there's no coercion here.