I don't think there's anything else you can or should say. My husband brought up the topic without actually saying, "would you?" It was more, "I heard about this swinging thing, what do you think of that?"
From there we talked a lot and I researched a lot, especially on this board. So she might be interested in reading these forums to learn more about it. Really, from your description it sounds like she is just not interested in the two of you having sex with other people. There's not a lot you can do to change that. I would keep enjoying what you have together and as everyone will tell you, "don't push her"