Castiel couldn't do much, but as Lucifer spilled over inside him, hips jacking as he rushed into his orgasm, Castiel managed to get through the gap in Lucifer's control. In return for Dean's prayer, he offered a sudden rush of platitude and comfort, peaceful reassurance, as though to say: 'I know, I hear you, I love you too.'

But that was all saying one thing while doing another. Castiel could have thrust Lucifer out, but he didn't--besides, where would Lucifer go if he did? Certainly up to Heaven, or into Sam's dreams and nightmares. He was as much a prison to Lucifer as Lucifer was a prison to him. So as much as he reassured Dean, he ruined him too, spilled hot come inside him, kept thrusting in an effort to pull him, too, over the edge.