Nothing concrete here, but I would suggest getting to know the other people you think about before you make a rash decision. It doesn't have to evolve into anything, but if you really love your current girlfriend, the best solution is to get to know the other girls to the point where you understand that cheating (probably) isn't the best idea.
Considering how far you and your girlfriend have gone, and considering you're still together, it would be a good idea to quit while you're ahead, aye?
If for whatever reason you can't justify staying with your current girlfriend, it's going to save everyone a lot of time and pain if you break up with her first.