You have to cum, and you´re going to

You decide to go for it. There was no way of talking yourself out of this situation, no lie could explain this, so why lie? If you are correct about him being turned on by this, even if its only subconsciously, you could use that.

"Dude" you say, looking at him accusatory. "I was about to cum. Gonna let me finish or what?"

It works. Ken seems so taken a back by the situation, that he just remains silent, his eyes glued to your cock, and his girlfriends hand around it. You no longer pay him attention and look down to Carli.
"Go on. Get your reward like the good girl you are. You deserved it, the way you took my cock earlier."
Carli hesitates, but no longer than a second. She glances over to her boyfriend, then back to you. Then she begins to suck you off again.
One of her hands cups your balls, while another grabs your dick by the base. Her tongue twirls around the tip of your cock, she gags on it and tears run down her eyes whenever she gets a bit to deep, but she doesnt stop. It doesnt take long for you to get close to cumming again, which Carli notices.
She removes her head to jerk you off, her hand is fast. "Please cum for me! Please give me my reward daddy! Give me every last drop of you cum, I worked so hard! Pleasee!"

And then you cum. You feel your whole body tensen up as you errupt, first over her face, then in her mouth. She greedily sucks every last drop out of you. Only after you feel like theres no more cum inside you she gets u, her face covered in your cum, catching her breath. Carli swallows, and smiles at you. "Thank you."