I think unfaithful was the keyword, if it is consensual and open then the partner wasn't unfaithful.

I'd assume GP would only hold this judgement for someone cheating, not swinging.

The main point was trust. Trust is earned.

Whether you break a promise with your life partner or your business partner, you've destroyed a chunk of your reputation.

But how do you know it was unfaithful? Lots of open relationships don’t broadcast this fact due to the stigma and taboo around it. Plus, OP said they’d go on hearsay.

So the cheating got around ears but the open relationship wouldn't? What a strange selectively gossipy community. I'd think people would normally cover up both as they are personal, but all shit can get out.

it's not very hard to imagine a scenario where the extramarital activities could leak without the knowledge of the open relationship (e.g., someone sees something they weren't meant to see).