At around the same time, I also read Asimov's I, Robot. There was decidedly little in the way of sex there, yet it was unquestionably as mature, content-wise, as Lee's book.
Here's the thing: There was not, nor should there have been, any sign saying that either book contained adult content. I was capable of reading the words, and handling their content, on my own. The content that's labeled in a bookstore is the "early reader" and the "young adult" stuff, not the stuff aimed at adults. It's assumed that, if there's no label, the works are "adult," and (further), that readers can make their own call about how old they have to be to handle the stuff.
I'm assuming that all of your journals contain adult content, in the sense that intelligent people putting words on (virtual) paper tends to be adult content unless explicitly directed at kids. I don't need, want, or benefit from labels, and don't really have much use for anyone who does*. If anyone under fourteen comes across this, or any, journal, they should be able to make the same decision about reading stuff online as they would if they happened across Dunn's Conundrum in their local bookstore. If they're not capable of making such a decision, that's a problem that rests on their parents and teachers.
So, yeah. Livejournal's Mature Content idea is one that can go fuck itself. In the ear.
*I make an exception for adult-oriented images, as there's not the same ability for people to process the type of content before having seen the pictures themselves.