Well, I don't tell people with whom I merely associate, but are not friends. However, if I were monogamous I wouldn't share details about the relationship either.
A lot of my former coworkers (I just changed jobs) with whom I became close know. It was a big shrug for them. My mom knows, but she had poly tendencies herself. I have other family that learned about it directly from me after after a cousin decided to spread rumors. I told them straight out. Decided they could accept me or not. They accepted. Shrug.
I guess for me it depends on the degree of closeness - but as I said, I wouldn't talk about a mono relationship with most people either.