Lets suppose a daughter suspects her dad is fooling around with another woman but mom don't know. The daughter reads the diary and the dad walks in on her reading it. Heres the point...would a daughter have the right to do this, as her dads feels he has a right to read his daughters diary?
The logic here is faulty. We cannot reverse the roles and see if the same action is objectionable because the relationships are not equivalent. The father is directly responsible for the welfare of his daughter. The daughter is not directly responsible for the behavior and decisions of her father, regardless of their impact on her life.
As a mother, I'm with Yeru on this one. The bottom line is that these are MY children, and I will use any means or information at my disposal to keep them safe. If that means that I search the room of a child I suspect is using drugs, I'll do it in a heartbeat. If I see evidence that my child is not revealing something that could endanger their welfare, I will read that diary with no hesitation. That's part of my job as their mother.
To protect your children by close supervision or occasional breaches of 'privacy' and to lovingly allow your children to grow and make decisions are not mutually exclusive activites. I get frustrated with the mentality that our children should be our 'friends'. I have seen little that is more destructive to a parent/child relationship than regarding your child as your social equal when they are absolutely not. I listen to parents say "I can't control what he/she does". Bullshit. You are the PARENT, you not only can control their behavior, you must.
Your reward for maintaining a loving parental relationship using understanding and discipline while still allowing them to make their own choices is that when your child grows up to be your social equal, you can enjoy a friendship with them. I absolutely count my parents as two of my dearest friends, and am so thankful that they were proactive when there was trouble in adolescence. I may not have liked them much at the time, but I dearly love them and respect that they loved me enough to let me make mistakes, but also to find out what I was up to before I made decisions that could have had dire consequences.