Another maxim you'll hear from me often: "All cheaters lie." Don't expect your wife to be truthful with you when it comes to matters pertaining to her affair for quite some time. So asking HER to confirm her own contact is fruitless, and counterproductive to the overall idea that I'm trying to get you to see, and that is to begin to exhibit LEADING behavior.
Canceling your "date" for tonite is proactive, and leading, and it lets her know that SHE no longer controls the agenda.
You can simply say "I'm sorry, but I need to cancel for tonite." When she asks why, just say "I have a lot to think about, and I'm afraid I wouldn't be much company right now." Be polite, but not forlorn or "wounded puppy" at all.
Shedule no future "dates" with her. If she tries to schedule something before you can have "The Talk" with her, politely decline, using similar words as above.
btw, strike the word "ultimatum" from your vocabulary. It's controlling. You are NOT going to be telling HER what to do; you are going to be letting her know what YOU are -- and are not -- willing to tolerate in a marriage.
"I cannot control you, nor can I make your choices for you. All I can do is let you know that I am NOT willing to live in an open marriage, nor do I approve of you dating other men while we are still married. I felt you should know that, in very clear terms" is your position.
