Have you considered that maybe you are not so aroused by your therapist knowing you are masturbating, but are fixating on their approval and validation (something I am sure you have struggled to find and thus drawn to this new open dynamic). I do caution you to be careful as sometimes a new fixation can negatively develop into a fetish or compulsion. Maybe discuss with them how you are now developing this fixation and that it has guided your actions into a stronger behavioral pattern than previously discussed.