Just knock one out here, clear your head.

You can feel the heat in your face and the buzzing sensation that comes from intense arousal. Your skin goosebumps as blood leaves your extremities to flow southward. Ugh. Outside, it sounds like Jack and the girls are making their way into one of the bedrooms.
You look down at your dick, then over to the bottle of hand lotion on the sink. Sighing, you grab the bottle and slick up your cock, rubbing and stroking as you close your eyes and focus. With no porn to stimulate yourself, you listen to the sounds of female pleasure coming from outside the bathroom door. Ana is giggling and moaning by turns. You don't hear Lucy: maybe she's polishing Jack's prick. You stand up, creeping to the door of the bathroom and stroking as you go. You crack the door, and are treated to the view into the adjacent room.
Lucy is naked except for blue panties, and she does indeed have Jack's cock packed into her mouth. Her cheeks are puffed like a chipmunk's, and Jack is aggressive guiding her by the back of the head with one hand while he fingers a stark naked Ana, who's lying on her back on the bed, legs spread-eagled.
You stroke harder, watching the illicit scene. You feel a little shame at the fact that you're using this scene to get off, secretly, but don't have the balls to join in yourself. The shame fuels your lust in an unpleasant way, but you can't help yourself as you lean against the doorjamb and work yourself.
Eventually, Jack begins to fuck Ana, while Lucy strips out of her panties and crawls onto the bed next to her. She presents her enormous ass to Jack, who obliges her with a few fingers while he fucks the other girl. You pump harder. He trades back and forth, always fingering the girl he's not fucking. You feel disgust. Karen is sexy, and a good woman. Why is it so easy for him to cheat on her like this? On the other hand, why haven't you just left?
As your jacking intensifies, the girls put Jack on his back, and Lucy rides him while Ana sits on his face. Before you can slink back, Lucy's eye catches yours. She can see what you're doing. The smirk on her face tells the story: pity and amusement at your voyeurism. She makes a little kiss face before riding Jack with more fervor. "Come on, Jack. Fuck me good. Like a real man." She says loud enough for you to hear clearly. The humiliation should encourage you to stop, but it just fuels your arousal more, until your furiously stroke out an orgasm, catching the cum with your other hand so as not to spill onto the floor. You stumble back into the bathroom, leaning against the wall and panting. What is wrong with you? You're so filled with shame and humiliation... but it was hot.
As the sounds of the sex continue across the hall, you put away your dick and scrub your hands and face in the sink. When the time is right, you slink to the laundry room and retrieve your still-damp shorts. You leave sheepishly, not bothering to let them know: from the sounds they're making, they aren't worried about what you're doing.
You drive home, red faced and sweaty, and you pull into the driveway around 2:15 pm.