He's taken somewhat by surprise; his concentration had been focused on getting the waistband past Crowley's hips (and off, if possible), but when he's so willing to be distracted this way, the grand plan is easily modified.

He takes his time; the kiss is relaxed and thorough and draws a tiny, pleased sound out of him. That furtive hand, in the meantime, is creeping downward again. And this time there's quite a bit less cloth between him and Crowley.