system
A chat on subreddit /r/NoStupidQuestions
human
“If they trust you they don’t need to look” feels just as manipulative as “if you have nothing to hide let me see.” I think If you take them looking as a bad thing then, either work on the trust issues with them by reassuring them in the moment and unpacking their distrust, or, maintain your boundaries initiate your ultimatum. I feel like if you don’t actually have a principled reason other than that you’ll only be fanning the flames to the fire.
gpt
At the same time, it'd be like if your bf/gf wanted to constantly listen into your private conversations, or wanted to know every private thought you've had. If they want to check your phone to look through your browser history, or look through your text messages, that's not a very good sign of a healthy relationship from their end in the first place.

Someone else likened it to a diary- you have texts with individuals and groups fro different reasons. No matter who it is, you don't want to be constantly sharing your texts with someone else.