I am someone who is in the same boat as your girlfriend. My wife dropped the bomb on me last weekend, made it clear that she is will consider divorcing me and shattering our young children's otherwise stable home environment if I didn't let her pursue a poly lifestyle sooner than later. Keep in mind that my response is from the POV of someone who feels like poly is being forced onto them against their will before I've had a chance to digest what it even entails.
You have to understand that poly isn't for everyone, and your girlfriend may be one who will never "get it". You have to acknowledge her feelings and beliefs - let her know that you understand and sympathize with her. Bring her along slowly. She will ask some tough questions of you, and you have to be prepared to answer them (and not give her the "that's just the way I am" speech - If you want her to spend the time and energy analyzing her jealousy and insecurity, you'd better prepare yourself for a LOT of introspection on WHY you have to have this). You will fight and argue. It may get to the point where it seems that is all you do (and if this means that much to you, make sure to stay in but do not get angry, and acknowledge her feelings). If she's like me, she's terribly hurt and confused and feels pressured to accept this foreign concept (to her).
In the end, like my wife may have to do, you may have to choose what is more important to you: a poly lifestyle or her.
I hope this was at least a little helpful, and sorry if its not exactly what you wanted to hear (and please forgive me for rambling - I'm a member of the walking wounded searching for answers on here).
Last edited by boba; 05-25-2012 at 04:26 PM.