You get aroused.

Much to your amazement, instead of getting angry at Mary, you seem to be becoming... hard! Man, watching your girlfriend act like a total slut to raise her grade... it's like a bad porn movie! Quckly, you fiddle with the controls, and turn on the sound.

Mr Woods: Oh baby, you do that so well... mmmm.

Mary: (disgorges his cock for a second, keeps jerking it) What kind of grade will this get me, mr Woods?

MW: Well... unhhh... I'll give you a C+ for the blowjob alone, and a B+ if you manage to swallow it all.

Mary: (Back on his cock) mmm.

You watch, cock in hand, as Mary expertly works over his hard rod, bringing him to climax inside her mouth, and not failing to **** down a single drop.

Mary: Mmm... yummy.

MW: Oh, shit. That was amazing. B+ for you... unless you'd care to go for the A+?