It's comfortable. Strangely so, considering Castiel's usual aversion to anything that limits or threatens his ability to breathe now that he needs in, after choking on souls and water and darkness. But this is safety, this helps Castiel calm down, because it's just another way for Sam to fill him, to claim parts of Castiel's body for himself, and take care of what's his thoroughly.

The slow pace allows Castiel to lose himself in the act, keep his throat relaxed for Sam to push into. After a while his muscles relax further. He repositions himself, subtly changing the angle, and looks up at Sam. Blinks slowly, given that he can't nod right now. Gives his go-ahead.