Strike first...

You lay in bed, listening to Jenna snore softly, when around 1:00 AM, you hear the faint sound of someone climbing the stairs. The footsteps pass by your door, and you hear the door to the guest room open and then close.
Perhaps... you could take this opportunity to strike first in the new paradigm. It's true, you and Jenna haven't laid out the ground rules yet, but... why did she tell you about Karen's little crush. Did she not expect you to do something about it?
You quietly get up, clad only in your boxers, and you sneak out into the hallway. The floor is dark, and you can see dim light under the door to guest room, for a moment, before it winks out.
You open the door into the dim room. Despite the dark, you can make out the outline of Karen, laid out flat on the bed. She has the comforter tossed aside, and it seems that she's changed into a lacy white nightie. As you approach, quietly, your eyes focus a little more, and you're greeted with the pleasant sight of her butt: her skirt has ridden up and over her hips.
You gently lower yourself onto the bed next to her. She shifts a little.
"...mmm...Jack...?" You run your hand up her bare leg, fingers curled inside to touch her soft inner thigh. "...mmm... cut...it...out...jerk." You lean down to kiss her neck, just under the ear. Jenna had shared a few secret weapons to use on Karen, though you suspect she didn't expect you to use them so soon.
"...fffuck...you....not fair..." your hand climbs higher and higher on her leg, and she groans as she lets her legs widen, giving you more access.
"...doesn't change anything, Jack." Karen mumbles.
"Mmmhmm." you hum into her neck. Your fingers reach her warm snatch. You gently rub and probe, slipping two fingers inside. It's gloriously slick and hot inside.
"Oh..fuck...it...." she shifts up onto her knees. "Make it quick..."
You smile. She still hasn't figured it out. But you won't make it quick. You move around behind her, and start to eat her presented pussy with skill and tenderness, spelling out the letters of the alphabet on her clit with your tongue.
"Ooh! What?" she sounds confused. "Oooh....Jack...what the..." she reaches back with a hand to lay it on yours, which is resting on her hip. "Baaabyyy...." she moans out..."where'd you...leeearn...that...."

You continue to eat her pussy, and she continues to be bewildered at Jack's new found skill and focus on her pleasure.
"Jesus..Jack I'm going to cum..." you get the sense, from her voice, that Jack's never made her cum with his mouth before. What a chump.
She flips onto her back suddenly. The jig is up as her eyes focus on your face.
"J...James?!?" she gasps in shock.
You climb up quickly to put a finger to her lips. "Shhhh.... don't wake the whole house, Karen."
"But..what the hell..." she's still panting. You didn't quite get her there, and the hunger in her eyes is unmistakable. "What possessed you... Jenna...how could you...?" She cuts off as you finger her pussy.
"Don't worry, Karen." You kiss her neck spot, and she moans involuntarily. "Jenna told me...about you and Jack...and about your fantasy. She told me to make you happy." You fondle her big tits as you rub your concealed cock against her wet entrance, soaking the fabric.
"Why...why would she..."
"We're trying an open thing..."
Karen chuckles... "Oh...I guess you found out about...what happened, then." She sighs.
"Yes. You knew?" You try not to get angry.
"Yeah, she told me about it. She was a wreck on the phone. I though she was going to hurt herself." Karen's eyes squeeze shut, and she shudders. "Get rid of those boxers, Jameson. If you're just going to tease me, then get out."
You pull down your underwear, wasting no time in plunging the tip of your dick inside her cunt.
"Ooooh....I can already tell...you're bigger than Jack." She grips your head in both hands.
"Well... like I said...we're going to try an open thing..." You slide further in as her eyes get wider, and then pinch closed.
"Fuck...James...that's...it's been so long. A dildo just isn't the saaaamme....."