Harry mouths Severus's hardening cock through the thin fabric of his trousers. He marvels at the hitch in Severus's breath as he grazes the side of Severus's cock with his teeth.
"Stop teasing, or you'll be punished."
"Counting on it," Harry murmurs before tugging down the waistband of Severus's trousers with his teeth and cradling Severus's balls with it.
He starts at the base, licking his way up Severus's cock with the flat of his tongue. At the top, the point of his tongue flicks across Severus's slit, darting in just so, before Harry's lips stretch around Severus's erection.
Strong hands fist his hair, pulling, forcing him down. Harry gives in and takes Severus in as far as he can go.
"That's it, take it you little beast. Take it all," Severus chants, fucking Harry's mouth.