For what it's worth, no one in my family knows and I'm okay with that. I was with my primary for about three years before we opened up, and she was very much aware and involved (if not part of) my family. At that point, I considered my options in that sense and decided there was no reason to bother with it. Should I ever get in a secondary partnership that is so involved that I would need/want them at a family gathering in a more-than-friendly way, I would deal with it then.
In other words, for me it's sort of a matter of worth vs. effort.
As far as people outside of my relationship, it comes down to people that I trust. There are a handful of friends that know my situation, who have varying degrees of closeness. Unlike SchrodingersCat, I do feel a need to be out to be honest, so I have to consider how important that is. I basically tell people when I really feel the need to and feel safe to (to get feedback, or to avoid sounding like a total creep, or even just because I want to be closer to them). My only real criteria for telling someone is whether they will be discreet enough; I don't want people that might not be ready to accept my life to write me off, so the people I tell need to be able to differentiate between appropriate times/ways to talk about it.
The hardest part about opening up was maintaining a sense of safety and identity for my primary relationship while still being honest. She has more backlash to face than I.