There are those who are mono in poly dynamics. My boyfriend is mono. He's free to have other partners-but he doesn't choose to and likely never will. There are others like him.
On the other hand, my husband is more the type who wants it to be "equal" all around.
Really the best thing to do is to consider in yourself what it is that you feel threatened by and why do you feel it's a threat.
Then address that within yourself.
Personally-the whole thing you laid out breaks many of hte poly boundary rules we have in our relationships-but EVERY SINGLE POLY DYNAMIC creates it's own "norm" so if she's not breaking boundaries-it's to you to figure out your limits and ensure that you are meeting them.