Sometimes Dean forgets how old Cas is. Because Cas is newly human, and therefore newly alive to everything awesome about being human, Dean tends to revert to big brother mode when he's introducing Cas to things. It feels wrong to push Cas about sex, to be anything but slow and gentle.

It always shocks him when Cas has other ideas.

Like when Cas pins Dean's wrists, bites his way along his oversensitized flesh and holds… right… there, ignoring Dean's curses and pleas, making Dean remember all at once that Castiel has commanded more power than Dean has ever dreamed of.