You are not alone
I'm in a somewhat similar situation. I was blindsided by my fiancée wanting to explore outside relationships (something she feels hardwired to do). I had very similar reaction as well, but my advice (about a week since the topic was raised) is continue what you are doing now.
Seek information. Read the stories people are telling here, and elsewhere. But most importantly, ask her what it is she wants. Try to keep in mind that it is about the pursuit of happiness. But not just hers, yours as well. Talk to her and figure out what can be a compromise that might keep you both in your comfort zone? And clearly let her know you need time to understand.
Talk, and slowly whittle away the fear. Ask her to help you understand.
I've found that during our talks about this, we've grown closer than we have in months. We are trying to figure this out safely, respectfully, and most importantly, together.
See it as a chance to bring a new level of transparency and honesty between you.
Clear your thoughts (I found that writing a letter over the length of a day focused my concerns and feelings immensely). And be sure to express them. Help her see your perspective. She has probably had months to think about these feelings, you have not. :-\
Progress and compromise, one step at a time.