The main reason I became polyamorous is this: I am an adult, and I exclusively date other adults. Adults are able to make their own decisions, and run their lives as they see fit.
Invasion of privacy to me comes off as disrespectful and controlling; and as though I'm being treated like a child.
In general, I'm a fairly open person. If someone wants to know something about me, I'll probably tell them. If I don't, there's probably a reason for it and I'll explain what it is. If that explanation isn't enough, they can try to convince me. Doing it against my will, however, I can't tolerate.