A Secret Desire at Work

I’m a barista and I’ve had this secret for months now. Last night, while my partner was away, I found myself in the back room with another customer, let’s call him Alex, he's forty, and we ended up roleplaying a scene where I was his assistant. It got pretty intense, very graphic, more than I ever imagined could happen at work. I feel so ashamed and don’t know if it’s normal to want something like this when you’re already in a relationship. What should I do? Should I tell my partner about this or just keep it my secret?