Join them.

Jenna's starting to work fingers into her snatch now as she takes Jack's dick into her throat.
"Fu....fuckkk..." Jack is arching his back, eyes closed now.
Jenna's bikini bottoms slide down to her knees, revealing her pussy and pucker to you. She's working fingers furiously in that cunt, getting ready to take Jack's dick inside it, you suppose.
You decide that's not going to happen...yet.
Quietly, you slip into the room. Jack's eyes are welded shut, and Jenna's too focused on the task at hand to sense your approach. You let your shorts drop, stroking your cock as you get closer to the bed. You manage to make it all the way to Jenna before Jack's eyes open, noticing you.
"Oh, shit!"
But it's too late. You climb behind Jenna's ass and slide your rock hard cock into her cheating wet pussy.
"Well, well well..." you say as you pound Jenna's snatch. She's squealing and moaning as she pulls off of Jack's dick to look at you with a face somewhere between embarrassment and desire.
"James..." Jack doesn't know what to say or think right now.."I..."
"You're getting your dick sucked by my wife, Jack. I think you may want to shut up while you're ahead." You grab Jenna's head. "Don't stop now, honey. That would be very rude to our guest." Her eyes blaze with desire for you as you push her back down towards it.
"Yes, baby." she resumes her furious deepthroating as you pound her pussy.

"Didn't take you long to find another cock, huh, Jenna? And your brother-in-law, no less!" You punctuate your words with savage thrusts, and Jenna whimpers against Jack's cock. "You're such a slut." You yank her back by the hair. "Say it!"
"I'm a slut.... I'm YOUR slut, baby..." Jack doesn't understand any of this, but he seems content to just ride it out.
"What were you going to do, my slutty wife?"
"I..." she stops to slobber on Jack's member some more.. "I was going to ride Jack's dick. Ride it until he shot a load deep in my pussy..."
"What then..."
"I was going to....ah ah ah ah....." she shudders as you pound her pussy furiously, "Tell you all about it, while you reclaimed me...while you showed me what a real fuck is."
You push yourself off her. "Well go ahead. Ride that cock, slut."
She hesitates only for a second before climbing onto Jack's dick and sliding it inside.
"Ooooh... Jesus..." Jack grabs Jenna by the hips as she bounces, then grinds, then bounces some more. You climb onto the bed, standing next to her, dick at face height.
She needs no instruction. She begins to suck your dick as she bounces on Jack, spitting and stroking and rubbing your balls as she stares up at you. Jack is a glorified dildo right now: her eyes are deadlocked on yours. After about 30 seconds, Jack starts panting.
"I'm...fuck...I'm gonna..."
"Oh no you don't." You reach down and drag Jenna off that dick before Jack can shoot his load. "Sluts get facials, right baby?" You hold her head over Jack's cock as he strokes furiously, and she nods as he paints her face with several shots.
"Ahhhhh....." the sight of Jack's cum on Jenna's face is so intoxicating. You throw her down and pound her pussy in the missionary position. You hit every special contour and spot, and Jenna cums in no time.
"Baby..."
"Who fucks you the best?"
"You do...."
"I can't FUCKING hear you, Jenna!"
She leans up, grabbing your head. "YOU FUCK ME THE BEST, BABY! OWN THAT PUSSY!" It's loud to your ears, but pitched low.
You cum hard, filling that pussy. After, Jenna pulls you down into a passionate kiss. You ignore the cum still sticking to her face. She's so beautiful...so sexy. The cum only makes her more beautiful.
"Wow..." Jack is pulling up his pants, shaking his head. "That was...."
"Yeah." Jenna replies without breaking eye contact.
"So you guys are into...some kinky shit now, huh?" Jack is having trouble catching his breath.
"Yeah." you say quietly.
"We.." Jack is standing up now, legs wobbly. "We should get Karen in on this. I don't think it would take much convincing...a little...swinging?" Jack's looking for a good way forward.
"Well I certainly wouldn't mind a crack at Karen. What do you think, Jenna?" She still hasn't broken eye contact.
...
...
...
...
...
"..Absolutely...."

(Achievement Unlocked! Wobbly H - Spitroast either Karen or Jenna with Jack)