I think your GF is certainly overreacting. What you did is wrong but understandable. In my view, anyone who swings under a prescribed set of rules needs to make small allowances for minor transgressions. When you are caught up in the moment you can get a little carried away, and as long as you do not fundamentally breach trust, you shoudl be forgiven. If you cannot allow for minor mistakes in the heat of passion, i think you would need to conclude that swinging is not for you, as a couple.
I compare it to driving. The speed limit is 60, sometimes you drift up to 65 - no big deal. But getting drunk and driving at 85 - big deal. Your GF needs to be able to distinguish between those two extremes. If she cannot, best stay off of the road.