Open relationships can and do work but they require significant commitment from all parties to the boundaries of everyone involved, which requires constant honest communication and emotional work. Your girlfriend might be naturally polyamory-inclined -- i was too when i was 18 and i still am. And if you don't like this idea, then you're probably not. It's not that this is something that you need to break up over, but it's very difficult for people with very different orientations in regards to number of partners to have a long term relationship that's fulfilling to everyone involved. I don't think it's impossible, but it would be hard, and I haven't seen it. You guys are REALLY young -- if this is a dealbreaker for either one of you, it's probably for the best.