I definitely agree about the distinction between honesty and privacy.
When we sit down to talk about how a particular relationship configuration is going to work for each of us - the time when we each talk about our needs, wants and likes for the relationship, and how well those are going to be met and respected by all, we also talk about privacy, to establish the expectations about what will and won't be talked about. Mostly this is about sex, but also covers other things.
I am not a fan of gossip - I believe that if someone has something to tell me, that is their story to tell, and it's not up to me to spread it around. So if one of my partners has some very important news, then I expect them to tell my other partner(s) about it, and to make sure that I know that they know. If I do not hear that, and feel that the news, or information or whatever is rather important for the other to know, then I will strongly encourage/suggest that the person be told.
My relationship style lends itself to this, and others may not have this - it's important to me that everyone involved be able to communicate with each other, so that I am not the "go-between" when any issues arise.
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