I'm not a guy, but I just thought I'd answer & say I wouldn't care if a boyfriend went through my phone, except I'm really anal about ANYBODY looking at my phone. It's not that the guy's are hiding anything, they just want to be trusted more.
im not a guy but i still don't think they would appreciate that very much. Its a little something called privacy.
There are just some things that girls shouldn't know...
besides if u guys haven't been together for that long a time, then it makes it more obvious that one doesn't trust another if they check eachother's phone.
it represents a lack of trust. and trust is key to a good relationship.