You let her have her way with you.

You watch as she bends down, her tongue trailing your abdomen, finally ending at the base of your erect penis. Then, positioning herself so you have a perfect view of the action (not to mention her bouncing breasts), she runs it up the length of your member and wraps her lips around the tip. There she stays for a moment, the tip of her tongue teasing your own tip. Then, slowly, very slowly, she takes it into her mouth. Her eyes never leave yours even as you feel it touch the back of her throat. Then, still maintaining eye contact throughout, she wraps her tongue tightly around your penis while it still sits in her mouth. To top this off she grips down with her jaw, making a cage of sharp teeth which would have no doubt injured a mortal penis. When she eventually pulls up, the pressure is incredible and you almost come right there. Behind her you see Alex stir and saunter towards Karen, whose vulva sits exposed, beginning for attention. He lifts up her dress, which gives you a lovely view of her perfect buttocks and slender back, and enters her unceremoniously. You feel her shudder through your penis as Alex begins to pound her. She closes her eyes, momentarily forgetting about you. You can't have that. You grab her by the chin and her eyes shoot open, an expectant smile spreading across her face. Then, your other hand grasped painfully onto her hair, you shove her down onto your cock. You stimulate your self using her mouth like a fucktoy, neither of you ever losing eye contact with one another. She keeps up the pressure with tongue and teeth, it becomes harder than ever to hold onto your orgasm, but your undead willpower is immeasurable. Alex pounds harder and faster, so you respond in kind. Then, when you feel like you are about to burst, Karen lets out a yelp, as you come together. You shoot a massive load out into her mouth as she quivers and writhes, spilling your cum on the floor. You manage to keep your dick in her mouth, even as she bucks against you, her eyes rolling back into her head. Then, you pull out, fully complete, and stand by Alex, admiring your handiwork. Karen still quivers, one hand clutching uselessly at her labia, the other over her mouth, trying to keep all your cum in. Shortly, she stops, and with a heavy gulp, swallows the last of your seed. Then she smiles up at you, back to being an angel. You offer her a hand, a mere courtesy, she could leap up without the use of her own, but she takes it. There is a slow clap behind you and you turn to see Gwen walking gracefully towards the three of you.
"Very good display, I was almost tempted to get involved. But now we have business to conduct."