First of all, my opinion is that you can't redefine someone's existence. It sounds like you probably met her at a party. Now that you're her man, why should she stop acting like she did before she met you?
Also, what is she to you? Marriage material?
If not, you might consider just how open you are willing to be...maybe it's time to try that MFM 3-some she's been hankering for...or that FMF or FFM for your desires! All 3 are loads of fun with the right chemistry. And you can learn a lot from other lovers' techniques.
One of the best compliments came from my wife and her friends when they explained that I was better than any of their female lovers.
And the second from my friend: dude, I thought
I was an awesome pussy licker, but you are fucking amazing! Master...
You already hinted that you were up for her and another woman, so return the favor. This could turn into a beautiful thing: just think, your girl and guy friend tag-teaming the hottest ladies at the party and bringing them home for the 4 of your enjoyment!
If you don't want to walk on the wild side during her (your?) college years, have you had a monogamous discussion with her yet?
And lastly, if you feel that strongly about the whole thing, you can clip her wings: fuck the ever living hell out of her so she couldn't possibly think about taking another man
Oh, I forgot to add that I disagree with the idea that partiers don't like stability. I personally crave it. What I don't like, though, is insecurity and constriction. Asking her routinely what she's up to when you're not around and other men in her life, & etc. is a surefire way to turn someone who wouldn't be opposed to some weekly grounding into reconsidering your overall value to her life at this point.