Well "need" and "want" sometimes break down into a semantic discussion. I think the point in this is that M realized that a poly life would be a good thing for her ... and by extension, for her and you as well. It's an opportunity for the two of you to grow closer as she shares her experiences and desires.
Ultimately, this is something she wanted, and you've made the leap to be okay with her about that. That is an amazing gift for a spouse to give, and I commend you for "leaping that chasm." It's not supported by society at large (yet), so it takes a great deal of introspection and inner strength to be okay with this whole "poly" thing.
I hope you guys will have many happy moments to share in the future.
Love means never having to say, "Put down that meat cleaver!"