There's no resistance as Sam pushes into him, and the only tension if of the good kind, the kind Sam no doubt desires to see in Castiel, the tension of him fighting to hold still for Sam instead of getting impatient. Sam sinks into him with ease, fills him, floods him with sensation and the very mild and very welcome burn of the initial stretch, that makes Castiel's head spin. He relaxes once Sam is fully nestles inside of him, as deep as he can possibly slide into Cas, and the fallen angel just breathes out, sighs in bliss and forgets about the rest of the world for here, for now.