If one partner isn't totally ready, and it sounds like she isn't nearly secure enough yet, you both should step back and talk about it more. In theory, you and her should have the same rights in this relationship...what's good for the goose is good for the gander as they say.
My hubby was first to foray into poly with my full consent, but I told him from the start that I reserved the right to do the same. When I did exercise that right, he found it rather hard...but good communication and respect of the other's feelings help. Good luck