How Do I Tell Him?

So a little bit of a back story:

(This went on almost every weekend for about seven months)

I was paired with a coworker to investigate a couple. (We work in law enforcement) and in their home, they had their own Fifty Shades of Gray Red Room. On the way back to the office, I found out that he was into BDSM and offered to be my Aladdin and show me the world of BDSM. after a bit of hesitation, I took him up on the offer and we started immediately. It started with little things like ice cubes, wax, spanking, etc. And then we worked our way up to my necklace. We would go out and it was like our tell tale sign that I was his and only his. Typing it out makes it sound less hot than it was. But anyway, we had an agreement that we would never, ever kiss on the lips.

Then one night, we had a moment. There was no spanking, no toys, no pet names. It was slow and sensual and it all started with a kiss on the lips. I always found him attractive and now I’ve fallen for him. But the problem is that we both promised that this wouldn’t happen.

Am I supposed to tell him? I was thinking of just transferring to the police station back in my hometown and pretending this never happened. How would we tell our boss if he feels the same. Am I stupid for thinking this would happen without someone getting hurt?