She leans back, considering this:
"Well, then we'll see how you feel about it after a week of Kara's teasing!"
You ask Erica to wait while you think about that. You think for a bit, then you ask if she will really end the relationship if you cum while Kara is cock-sitting you.
"Yes, none of that changes, but you can agree to be my permanent tease toy now and get to fuck me, right now! And then we won't have to worry about Kara's teasing this next week..."
The offer sounds better all the time, you admit. And Erica looks so good, you can barely control yourself.