I am new to the polyamory world too. I am currently dating someone with the intent on opening up our relationship to other loves. I understand how you feel - every time Ourobors mentioned this idea earlier it freaked me out. All the thoughts you posted ran thru my head as well.
If she is not considering exploring this right now, just take some time to think about it for yourself. There is no need to force yourself to believe one thing or another, just sit with it for a while and talk to her when you feel comfortable or when you have questions.
Time and patience will probably reveal alot to you both. There isn't really any need to rush, especially if you feel uncomfortable right now. But remember to keep an open mind, and try to develop your own, independent thoughts about this. It is best if you come to an opinion or conclusion based on YOUR true feelings, not because of someone else - although they might open your eyes to begin with.
The goal, to me anyhow, is to feel centered in my decision and to be sure I am looking out for myself at all times.