I would think marriage is a public matter.... But that misses the point.
If more than 3 people are aware of some given information, all people aware of that information should assume it is not private, and the world knowing is outside of their control.
From my point of view, the "right to privacy" would be akin to the "right to bear arms". It does not mean anything if it is not exercised. In other words, no government gives people rights to "bear arms" or "have privacy", the people who care enough will exercise the responsibility of obtaining such abilities.
Well, that's every right the government grants. A government-granted right is essentially the government saying, "we won't stop you from doing X".
Anyway, I think your definition of privacy is a little stringent.
If I am required, by law, to disclose that I am doing activity X in my home, with some kind of penalty imposed if I fail to disclose my performance of activity X, then that's not private. I have no choice whether people know what I'm doing in my home.
But we have the opposite situation. It is illegal for anybody to come into my home without my permission. Even the government and law enforcement officials. Therefore, anything I do in my home is private.
Sure, I could tell three or more people what I'm doing, but that's my choice. I control who does and does not know what's going on. It's my information to disclose or withhold as I see fit. So it's private information. If I'm really worried about it becoming less private, I can even have people sign various contracts not to disclose the information, which imposes a penalty on them if they don't maintain my privacy.
If I control the disclosure of the information, then it's private information, until I choose to make it public.