Her asking that question makes me believe that this is doomed to fail soon, no matter what you say.
If you say you don't want to do the open relation thing, she will probably dump you anyway, I mean, she's already thinking about what it would be like to hook up with other people. She will say something like "I NEED SPACE

"
If you say yes, she will probably take full advantage of it and you will be left with sloppy seconds or the fact that she kissed/fucked other guys will gross you out and even if she's still with you after all this then you'll just be tormenting yourself.
Sure you get to screw around a little as well but you say you really like this girl, that won't be satisfying.
Your relationship is already over, and you should pull the trigger and you can feel good about yourself if you can be that strong, but thats just my 2 cents.
Ok i understand that were doomed to fail but thats ok with me because were going off to college in september and wed break up anyway but for the time being i still want to be together with her..so what do you suggest I do? thanks guys