You know when they say pick your battles? This is one of those times, and one you don't want to pick this battle. The Piss-Off Value is extremely low here.
You can't make the argument without looking like you're hiding something. You've already acknowledged this, and it's true. In her eyes, you're trying to hide something, and it's only going to make her want to do it more often and with more stuff (like texts, emails).You aren't being harmed by the opening in any significant way. Mail sucks, and you're just being petty about it. You already said she doesn't check your texts, emails, facebook, etc... Keep harping on this, and that might change.What benefit are you truly getting by opening your own mail? Answer---->Nothing, except some false sense of privacy. What detriment are you bringing to your doorstep if you keep this up? Answer----->suspicious wife, pissy wife, defensive wife. All bad things. All things you clearly want to avoid, ESPECIALLY when the benefit is so low.Start texting her from work and ask her to open your mail and give you a rundown of what is in it. Do it everyday for a week or so. She'll get sick of you asking her to do it and start saying "open your own mail".