But this turns out to have caused problems for my LJ friends, and perhaps I was giving myself too much credit anyway, so I switched back and said that my journal has 'no adult content'. I don't imagine that I will be changing this again anytime soon. Sorry for the confusion and annoyance this caused!
I gather that people were seeing two things as a result of my earlier decision, so I thought I'd give a quick summary of how to deal with those things in case you encounter another journal or post with a similar setup.
1) First of all, a lot of people saw all of the posts in my LJ as being behind a 'cut' tag. I believe that this is controlled by this page: http://www.livejournal.com/manage/settings/ which has an option 'Viewing Adult: Content Collapse adult-oriented material written by other users:'; if you don't want such material automatically put behind a cut this can be set to 'Do not collapse'.
People may also be interested in the 'Safe Search Filtering' option on this page.
2) Secondly, when people clicked on the cut, some of them were warned that they were potentially viewing adult content and were asked to verify that they were over 14 years old (or something like that). I believe that going to http://www.livejournal.com/manage/profile/ and changing your birth year to something that means you're old enough to see that kind of stuff will fix that. (You are of course free to lie, although as mskala points out doing so would violate LJ's Terms of Service and therefore could theoretically result in your account being suspended.)
I think that's about it. If you notice that I've made any errors here, let me know and I'll correct them. Thanks!
Oh, incidentally, my little project to go through all my old LJ entries and tag them appropriately is almost over -- I'm up to May of this year. People interested in browsing through old posts of mine can see all the tags I used at http://jwgh.livejournal.com/tag/ .