Color rises to Castiel's cheeks, and he pulls his hand away. It's not a jerking movement, and he doesn't actually seem worried or ashamed that Sam has seen - he merely seems embarrassed about having to explain.

"I enjoy being tied up during intercourse, Sam. It's not nearly as scandalous as human society pretends."

Still, he tugs the sleeve down, because he doesn't like the look in Sam's eyes.