I found this extremely relevant:
Personally, the issue of monogamy (and moving across the country to go to grad school where there will be a plethora of intelligent, attractive single people) is something my bf and I have talked about recently. We discussed the idea of going "open" for a set period of time, though we decided we're not quite ready to explore that outlet just yet. I'm sure it will come up again in the future, but in the mean time we've both been working on spicing things up and increasing spontaneity. A lot of people would be bothered by their partner telling them they've thought about a non-monogamous relationship, and people tend to warn me that my bf only brought it up because he's either cheating or tempted to cheat and just wants permission so he doesn't end up in the dog house. Pish posh, I say. Clearly these people don't know what open, honest communication is supposed to look like. Sharing our most intimate, secret desires with each other has greatly strengthened our understanding of and respect for each other.
Anyway, ah, that was a little off-topic I suppose. ^^; My point is, your feelings are totally normal. Ask yourself honestly, why do you believe the relationship will grow stale just because other options may be available? Has it already grown stale? You say "I care about my girlfriend and think highly of her," but the wording seems a bit weak and uncertain to me. How openly do you communicate? How does she feel about being "dragged along" when you go to grad school? Has she expressed any concern (or has she even considered) that either you, she, or both of you will likely be surrounded by new, interesting, attractive people? These are things you should be asking yourself (and possibly her as well).
Good luck to ya.