It's one of the few things that Sam knows won't set off Castiel's aversion to being unable to breathe. It's not something that Sam takes advantage of, always careful to only stay buried deep for a few moments before pulling back, letting Castiel breathe before pushing back in again. By now, it's something they both have experience with.

For the moment, though, Sam is content to stay still, head down to watch Castiel's face, see his eyes look up at him and his mouth stretched around Sam's cock. His hips move only slightly, not fucking Castiel's mouth but definitely moving to meet him. Though Sam absolutely will fuck his mouth later, once they're both ready. For now? It's that look and the concentration as Castiel does his best to take as much of Sam as he can.