To be fair, he did not actually spend the entire fifteen minutes jacking off. But he's not telling Dean that.

Instead, the water shuts off and Cas walks out of the bathroom with a towel and goes straight for his duffel. He looks more relaxed than he has in...ever, but he's definitely not meeting Dean's eyes and if his lack of conversation means anything, he's not talking to him just yet either.

...then again Cas doesn't often have things he needs to say anyway, so that could just be normal.