Not trying to tell you how to live your life, but being an open book to the other isn‘t very trusting. If your trust is build around being able to spy on the other, maybe you aren‘t trusting each other that much. My wife and I both have our own separate phones and computers without each other being able to access it and I trust her not to do anything that goes against our interests and vice versa. That‘s what trust is, not having to know what the other does and knowing they are doing the right thing.
I also let my spouse unlock my phone. It's not so much "being an open book" as it is "I trust her not to snoop, and sometimes it's convenient that she can open a map on my phone." That meets your definition of trust: not having to know what she does because I know she'll do the right thing.
Very fair point. In fact, my wife actually knows my phones passcode for the exact same reason. „Open book“ was definitely the wrong phrasing here, I wasn‘t trying to say that you have to keep everything secret from your spouse. Just that the exact opposite of that also strikes me as very distrusting.
There are also lots of people in abusive (or just "problematic") relationships for whom some privacy around their phone can be extremely valuable. That's why I'd prefer the societal norm to be NOT sharing credentials. Which of course doesn't mean that all people have to follow that.
I just got tired of having to unlock my phone every time I asked my boyfriend to send a message while I was driving, change what music was playing, etc., so I enrolled his fingerprint. I imagine most people that do this are thinking on the same lines... I would say that this is an indication of trust, as I trust him not to go snooping around on my phone.
Its kind of perverse to jump to spying, don't you think? I'm not spying on my wife or she on me. Even though I have her email on my phone, in the past year I think I've only been in there twice, both times when she asked me to look for something for her. I know she doesn't look at mine either because she just doesn't care.