The face Dean pulls can only be categorized as incredulous disgust. Like a feral hunting hound; the concept that he's an attack dog.

(The problem is that it isn't very far fetched, no pun intended.)

He can practically feel Cas vibrating out of his skin the instant the words are out of Geralt's mouth.

"Relax," comes the firm order lobbed in Castiel's direction, equal parts dismissive and reassuring. "Leashes aren't really my thing. I'm more of a cuffs and blindfolds guy."

No he isn't, but defusing the tension with a little irreverent bullshit is the Dean Winchester way.

And then to Geralt, a mild and completely unnecessary, "No offense. I'm sure you're great with those and all, but I'll leave that to Jaskier."