She wants a mono relationship with you, right? And you said you could imagine being in a mono relationship but can't promise her to maintain it forever. She wants to be with you anyway. Just to make sure that I get you right.
That seems a bit risky to me. Like one of you might have to change his view if this relationship should last. Either you give up being poly, resp. don't live poly, or she learns to accept that you might want to live poly one day. Maybe she hopes that you will feel fulfilled in a relationship with her, so that you don't want another partner anyway? Or she hopes that she can learn to deal with it? I think, you should talk a little bit more about that.
It's up to you to set your priorities. Does being with this woman seem more rewarding to you than living a poly lifestyle?
For me it wouldn't. In mono relationships I have always come to the point where I felt like I was lying to myself and the person I'm with, that I couldn't be 100% who I am and had to hold back some of the love I have inside of me. Moreover I see no wrong in being and living poly, so I see absolutely no need for me to live mono, no matter how much my partner would want me to.
So if I were you, I would make it clear to her that living mono with her would simply reflect the current circumstances that there is no other person I'm close with at the moment. And that I will build up equally close relationships with others when given the opportunity. To make the relationship work for her, she should neither hope that I might never meet someone again that I wish to build up a close relationship with, nor that I might change my mind about poly. But that's just how I would handle the situation.