"Doubt isn't always a bad thing." And he knows that Castiel knows that. But it still is a good thing to remind the former angel of from time to time.

Once he's certain Castiel is steady, Sam kneels and reaches for more soap, lathering up the washcloth again so that he can start washing the blood and semen off of Castiel's legs. Still gentle, still careful, but making certain every bit of it is gone. Including up over Castiel's ass because that definitely needed to be cleaned so that Castiel would feel better.