You're treading thin ice. I'd be careful, one wrong move could seriously fuck up your relationship with her. Have a straight up conversation with her about it. Say something like "Hey. You know, as well as I, that I've been hanging around with a lot of girls lately. You once told me that you want to experience high school to its fullest, so I assumed you wanted to try different guys. I'm happy to let you, as long as you let me try different girls as well. I want you to know that I still love you, and that I don't want to ever leave you, but this is just to confirm that idea. If, once you graduate, neither of us have found anyone better, then it proves that we are meant to be together. If not, I guess it wasn't meant to be." Something like that.