At some point, Sam goes from feigning sleep to dozing back off again. Not deeply, though, which is why he starts waking up when Castiel tenses against his chest.

He makes a tired sound against Castiel's hair, glad that Castiel comes to realize he's safe before Sam has to be awake enough to confirm it. "Hey, Cas," he murmurs, voice raspy from sleep. He tightens his arm, giving Castiel that little bit more comfort. "You sleep okay?"

It's not that he doesn't want to have sex with Castiel. It's that, right now, it would definitely not be a great idea.