Your girlfriend and Grant

You feel shocked as you open the door, you had almost forgotten that you even had a girlfriend for the last few hours as your head was in the clouds thinking of the party. You can't help but gasp as you see your girlfriend Sarah riding Grant's cock.
"S-Sarah what is going on here?", you ask, hoping that it wasn't what it looked like even though you can still watch her pussy wrap around his hard cock.
At first it seems like she doesn't hear you as her hip keeps moving quickly, her juices running down Grant's shaft. You start to feel angry as you hear her loud moans, you watch her ass move, slapping against Grant's legs over and over as she doesn't seem to stop. Her moans quickly grow louder and louder making you blush as you are unsure if she ever moaned this loud for you, you can't help but notice your cock grow harder at the view, soon forming a visible bulge in the front of your pants.
"Sarah please stop it", you try to talk to her and convince her to stop, feeling ashamed about getting turned on by the view. As she still continues to ride his cock you can't resist the urge to start rubbing your cock through your pants.