Castiel had not expected his orgasm to affect him in the way it did and certainly had not anticipated how hot Dean would look as he licked his cum from his lips.
Even someone as terrible at reading facial expressions as Cas could tell that Dean would be rock solid at that moment. Castiel had no real clue what he was meant to do with that as oral did not seem very Mastery.
"Show me" Cas heard himself order Dean, looking down pointedly at the bulge in his pants.