Dean considered oral sex to be one of the most vulnerable positions. He wouldn't trust many other than Cas to go down on him. It was a sign of trust and respect, despite the way Dean treated Cas outside the bedroom. it was in the bedroom that counted, as far as Dean was concerned.
"Fuck," he said again, hot and breathy. He was close, so close. Cas knew just how to work him up. His back arched and he rolled his hips against Cas, seeking the nudge he needed to topple over the edge in to complete surrender.