Basically, what I (or anyone else, for that matter) do with my body, property (incl. correspondence), and my thoughts is absolutely none of anyone's business - as long as I'm not getting in your way or limiting your freedom. And no, you're the one who needs to provide a strong enough argument for why privacy matters less than your need to know.
Individual liberty is the default and you're the one to provide a strong argument for why it should be done away with.
History teaches us that people who start poking their noses into what everyone is doing it never ends well.