Only you can decide how to respond to that. Your initial response was wise. Now come the big questions you have to ask. She has laid down an ultimatum. "I'm going to do what I want to do no matter how you feel about it." Is that a dealbreaker? Is it something you can tolerate for a fixed amount of time (a year, a month, or whatever)? Is it something you can put on hold until your next counseling session?
She is very caught up in NRE and is testing her limits. Her marriage is not coming across to her as being as important as it used to be. She is willing to take chances with it. Can you adapt to her new precedent of behavior? Should you adapt? Are you going to end up resenting her?
Maybe this is a one-time thing. But maybe it will become a habit. You can always wait and see what happens.
It's possible she thinks you *will* be okay with it, even though you said you wouldn't. You can let her test her theory and see what happens.
In any case, she's taking a heck of a gamble. I don't know what to advise you to do; your marriage, happiness, and peace of mind are at stake, so you will have to make a judgment call on which of those things can be salvaged and how.
Love means never having to say, "Put down that meat cleaver!"