My ex-husband and I had these issues. He would lock down the server (uber-geek, we had loads of computers in the house), and then something would inevitably go wrong with the connection to the printer, so I'd need the password----and even if at work, he try to log in remotely and fix it rather than just give me the frickin' password!
This extended to every aspect of our lives. Pin numbers to cards, I wasn't allowed to know them (he actually covered his hand even if I was only one around). He said 'if something ever happened, I wouldn't want to suspect you'. Well why would you suspect me anyway, jackass?
Forget about knowing passwords to email, there's no way I would have gotten in there. He would have insisted on printing things out himself or logging me in, no matter how inconvenient.
My current DP (soon to be DH) is nothing like this. I know his email password and pin numbers, and he knows mine. I normally don't go in his email unless he asks me to, and if I do it without asking I always mention it to him and he always says its fine.
I get what the OP is saying, sometimes you just want something that is yours. Having see it from both sides though, I infinitely prefer how things are with current DP. He makes me feel so trusted. The first time he gave me his pin number to his card, I was soooo touched that I actually started crying in the middle of IKEA. I hadn't even asked for it, and he just trusted me so much!
I guess what you have to do is think about how much securing your email is worth to you. Is it worth making your DH feel like you don't trust him?