Check out "And then there were three" in the Life Stories and blogs. It might be a similar situation to yours, although she is fine with him being poly. This shows you a possible outcome.
I was fine with my partner's polyamory intellectually as well and unfortunately I don't believe that you can understand the emotional ramifications until your partner is actively poly. At that point you can't really put the cat back in the bad however much you might want to.
Unless there are a whole heap of poly/mono relationships happily working away from forums that I don't know about the truth is that these relationships don't seem to work very well. You could be risking the relationship that you have. It's very easy to get caught up in the beginning with the excitement of polyamory. It's something new and fresh and exciting to play with. I love my partner very much and we have an excellent relationship but in many ways I wish we had never let the cat out of the bag.