He doesn't expect Castiel to be able to respond. After all, they've had sex enough for Sam to know that, when it's really good and when things are just right, it's almost impossible for Castiel to speak.

Which, honestly, is one hell of a compliment for Sam.

He pulls back, pushing in again slowly, movements steady though it takes so much for him to keep it that way. "You are mine, Cas. And I'm yours. Nothing you say or do could make me push you away. Because nothing can make me stop loving you."