In sport fencing, two of the most common parries are parry four, a middle line parry to the inside, and parry six, a middle line parry to the outside. Last night, I went to a rapier class and the only parry that was taught before we also learned to use the dagger was a parry four analog. During drills, I also reflexively used parry six, and was told not to do that. What is the reasoning behind not using an outside parry?
I'm afraid that you'd have to ask the people at the rapier class you attended, because not all HEMA practioners outlaw a Sixte. It is possible that they don't actually have anything against a Sixte, but you were practicing a Quarte, so they were simply chiding you to follow the drill.