I think they should not fear going through their partners phone. There should be trust there. Everyone is entitled to their privacy though and just because you are in a relationship, it doesn't mean you need to know every little thing about that person. Some things on a phone are embarrassing. It's not that they are hiding things or that it's a bad thing, but it's not something you want just anyone to know.
I'm comfortable letting my boyfriend see what's on my phone, and he has never acted protective of his. In fact, we both have each other's fingerprints stored on our iPhones so that we can access them easily. But I've never gone snooping through anything on his phone intentionally, and as far as I know, he has never done that with mine. But frankly, if he did, I couldn't care less. I have nothing to hide, and I don't mind if he's curious.
They should be able to trust each other enough not to otherwise their relationship isn't strong. They CAN go through each others phones though if it's a mutual agreement but honestly it's much better not to.
I would not be with someone who is super protective of their phone. I am around chill people which means they don't mind if I just grab their phone when I am board and heck even read messages/group messages to pass time. And I don't mind when they do the same.
so dating someone who is not as chill would be a torture and boring.
No, if you have to go through your partner's phone, it clearly shows that you lack of trust in your partner, and no relationship is complete without trust. If you can't trust that your partner isn't cheating on you, then you probably shouldn't be with this person.