Anyone who isn't doing anything wrong has nothing to hide anyway...AND, if you're with her/him because you love them, and you truly DO wanna be with her/him for the long haul, then let them check it. If you love them, you wouldn't be doing anything sneaky on ur phone in the first place. And remember, people who have "trust" issues and feel they need to check your phone, that's not an indicator of what kind of person you are, it's an indicator they were with a loser/cheater in the past and they've trust issues with EVERYONE. Anyway, that's my 2 cents
you should get to the bottom of WHY she doesn't trust you. Does she have a reason? Do you get mad when she does it, making her think you do have something to hide? Is she insecure? I sometimes check my husband's phone and he will check mine as well. We also know each other's email passwords. We have nothing to hide.