Before I attempt to answer it, the first thing I would encourage you to do would be about the TONE. You don't want to come across as angry, preachy, or vindictive. "Sad, but resolute" would be a good way to describe what I'd like to see you shoot for, as in "I'm sad that you've chosen this way to run away from our problems, but you're an adult, and I can't control you. I can only tell you that in no way am I willing to live in an open marriage."
If she accuses you of spying, I would suggest saying "Everything I've done, I've done to try to fight for our marriage and our family, but I certainly understand your being upset." or "I'm only trying to fight for our marriage." If she specifically uses the "privacy" word, be sure and make a distinction between "privacy" and "SECRETS". The best way I've seen it explained is "Privacy is closing the door when you go to the bathroom. Secrets is keeping something from your spouse that a husband and a wife should be sharing with one another."
While you will need to have an R talk at some point, and also from time to time, my advice would be to NOT get into it during THIS talk. This talk is about YOU -- YOUR boundaries of personal integrity, and YOUR stance. If she tries to talk about the relationship (and she will!), then say "I don't want to talk about that right now. I'm willing to address any and all issues -- including mine -- but not as long as you're having an affair."
And then don't waver from that.
It's fruitless to discuss the marital relationship so long as one party has unilaterally invited a third person into it.
