We both feel we don't have anything to hide (i.e., we know each others' passwords), but we try to respect the wishes of the other person when we do use what is "their" computer. My husband has had to use "my" computer a lot lately as his kind of died a few months ago. He has his own user account on my computer, but if he's just getting on for a little bit, he usually doesn't log me out and log himself in. Instead, he just uses Chrome and I use Opera to keep our internetty things separate.
I could care less if he has a porn collection, and similarly, he could care less if I have one. He is well aware of the fact I write erotica. That is the one aspect of my computer I prefer to keep private, and he respects that. He doesn't go looking for my writing.
A couple weeks ago, we had some friends over (very good friends) and our guy friend sat down at my laptop and just started using it, without even asking me
I told him he was one of the few people in my life who could EVER get away with doing that. Generally, if people are coming over, I hibernate my computer so someone can't get on without my password (and good luck to them if they can figure it out).
I guess what's more important to me is that we trust each other.