Briefly, myself and my wife have it pretty open. Most information, especially to do with computer accounts, etc., is there for the other to look at if they want to. Sometimes it's beneficial that the other person know what you've written to a mutual friend, so we check out sent emails that were written on behalf of us both.
That said, there are a number of people that would confide in me and request that whatever was discussed go no further than I. In this case, I would most certainly keep the information secure and take steps to ensure it remained as such.
BTW, my wife has a bit of a problem with me going through her wallet, where she keeps photos and a whole lot of other stuff, even if I'm just looking for a bit of cash or whatever (something harmless). She knows it is harmless, but is still uncomfortable. I think this is something to do with her family, and how everybody had their own wallets and it was pretty much understood that each person's wallet was private, no-go territory.
The key here folks, is understanding, and not necessarily an absolutely equal arrangement. I now understand why my wife needs a bit more privacy with her wallet, and to be honest, it doesn't worry me as long as I can get some cash or whatever if I need it. So now I just ask and that's OK, whereas before I didn't ask and it wasn't.
As for me, I couldn't give a toss if she spent 15 minutes going through my wallet each day. In fact, it would be useful because she could clean out all the pesky bills, vouchers, and other crap that I build up there over time :-)
Be cool and honest folks, and if you're not going to be honest, don't be dumb about it :-)