Also, how many couples actually keep their email secret from each other? How many would have no expectation of privacy here, and hence view the subject differently?
[0] - it happens, e.g. it's easy to accidentally mis-TAB when logging in to Facebook and have the password, appended to e-mail, saved in browser's auto-complete for login field.
Edit: My girlfriend and I have permission from each other to go through each other's phones/computers etc. and as a result of that we don't feel like the other person has anything to hide, and it brings us closer to each other. I have no interest in going through my girlfriend's phone/laptop, but if she refused to let me do that it would instantly bring up red flags that she doesn't trust me and that she can't be open with me. That's not the kind of relationship I'd like to be in.
Humans have a rare trait in being able to develop cyborg transparent interfaces with our tools. A hammer becomes your hand, as far as your brain is concerned, after you've used it long enough.
Phones and other technology do the same thing for our minds. The photos we snap but never review are memories we're trying to collect. Phone calls or IMs a better history of conversations we've had.
Someone else going through this technology w/o permission is like them having a way to psychically read your mind. Both are a violation of your self -- your identity, memories, what have you. It's a violation because just as we learn to use tools as physically indistinct appendages, so to do we use ways to capture our minds as indistinct appendages.