Caught Off Guard at Work

I've been in an open relationship for a while now and we're both okay with some casual encounters. One night at the bar, a regular customer made a move on me that I couldn't really say no to. It was just one time and everyone knows we're not exclusive but... something about it keeps gnawing at me. Is it weird that I feel like I betrayed my partner somehow? Or is this more about how casual flings feel so different from what we usually have together? What do you all think, am I overthinking this or does anyone else feel the same way?