I have to agree with Galagirl. You sound like two wonderful people who share different desires and needs in your life. Perhaps it is not possible to share a long term sustainable relationship with your girlfriend. What you outline would seem to work better with either a willing mono or another poly. I am curious...how would you feel if your gf wanted to explore love with others?
And I also see newtoday's points. If you truly want to be with your gf in a long-term relationship, can you see yourself staying commited to just her?
In my experience, if one does not want to be monogamous or is not wired for monogamy (like myself), than can you really stick to monogamy? Will you be truly happy never loving anyone else?
I think these are all important questions to ask yourself and your girlfriend. I wish you both great happiness, and if you cannot find it together, I hope you find it elsewhere.