That's new to say the least. If Dean was in any shape to think he would be wondering what Lucifer's end game was here. Surely he had some sort of plan and reason for doing this, right?

Dean gasped again when the second finger slipped in. He groans when those fingers curl just right and stroke along his prostate. He jumps, not expecting the feeling that courses through him. I mean yeah he's expiramented before but nothing like this has ever happened. "Cas..." Dean whined softly his fingers digging into the blankets as pleasure began to fill him.