I think there is a difference between willingness to share and demanding to know. My late partner had a business and a web-site. She used the site for financial info and even e-mails. I didn't care about anything on the site as it was her business. She would have shared if I asked, but I didn't. However, after she died a lot of important information was on that site and it as a real pain in getting it. You never know.
I see openess and honesty is critical in a committed relationship and that applies to both parties.

I think trust and communication is what makes a relationship... Privacy is about boundaries... so it really depends on the situation and the reasons for why you would give out passwords or not.