It does sound pretty straight forward, the sort of dream lots of people have actually. It's fairly normal for our minds to wonder what would happen if you were dating/married to a different person that we know, sometimes expressing thoughts that we don't allow to surface when we're awake. It doesn't necessarily mean it's what you want to do in your waking life, sometimes your unconscious plays out scenarios and thoughts just because it can - in fact our mind sometimes make us think about things we don't really want to when we're awake just because it can, which often makes us cringe.
Obviously you shouldn't dismiss it if you really do fancy the other person and/or want to leave your current girlfriend, but I'm just saying the dream isn't definitley a sign of this.