It doesn't happen instantaneously - it takes a few slams of his hips back towards Lucifer's for Dean to really spurn himself on. Until it's all over. His orgsm hits him agonizingly hard, coming with all that he has left across the top of the table. If he'd been able to make noise, it would've been a desperate groan, aching. But it's left rattling around inside his head, his hips still jerking with the aftershocks of climax, even as the rest of him goes limp.