Wow you guys are all a bunch of spineless wimps. You have to draw the line somewhere. You can't just say "Give her the password, problem solved" because that isn't the end of the issue. It's an obvious sign of the "Power Struggle" that relationships eventually turn to after the honeymoon stage is over. Ignoring the issue will only translate to more arguments over different topics without addressing the underlying issues of the state of the relationship. To OP and all the rest of you that just said "give in" You have to be able to draw a line somewhere and you are entitled to your own privacy and sense of self space. Having ZERO boundaries and barriers is not healthy for either partners and later on will develop into trust issues when one partner decided to actually draw a line in the sand for sanity sake.
OP I would talk to your s/o and just explain that you desire something that is "yours" and she shouldn't be trying to take that away from you simply because SHE wants it so.
For more info on the stage of the power struggle you can read here.
The Power Struggle
Berz out.