Can I enjoy an improv class without joining its end-of-course showcase?

At our last class, my partner for the exercises said he'd told his family we were doing the showcase together. I said I hadn't decided. He replied, 'You'll be fine. I'll do most of the talking.'

The teacher had described the performance as optional. I joined the course because I wanted a playful evening after work, and I've enjoyed inventing scenes with the same small group. Performing for their invited guests feels like a different activity.

When the sign-up sheet came around, I passed it on. My exercise partner looked disappointed and said his family had already arranged to come. I hadn't realised he had named me in his plan.

I'd like to attend as an audience member and keep taking classes afterward. How do I decline the showcase without making my partner feel I've abandoned something we were apparently preparing for together?