Do you think it's wise for a parent to read their teenaged children's diaries?

I personally think you’d be crazy not to. Now, what you do with the information you glean is a different story—your reaction shows if you’re a good parent, a mediocre parent, or a bad parent. Say you read that your 13 year old daughter is thinking of having sex with her 14 year old boyfriend. I think a good parent would take a deep breath, then patiently bide her/his time—days even, until the subject of sex came up “naturally”—then when there is an opening take the opportunity to share your thoughts on sex, birth control, decisions, the emotional impact, etc. A bad parent, at the other end of the spectrum, would freak out, take the diary and throw it in the girls face, scream that she’s a slut and whatever, and destroy whatever fragile bond they might have had left at that point…those are the two ends of the spectrum, IMO.