LOL. Well, the only problem with friends lock is that if people aren't on LJ (silly people) they can't see your stuff, yes? I think for the fandom world it's the difference between only being seen by your friends and having the possibility of being seen by others and widening your audience.

And I wonder about my own posts that have nudity in them... should I go back and mark those as adult content now? I have little warnings, but not major ones.

As far as the searching for porn when you're too young thing - this really isn't going to stop it. People are going to find it if they're looking, and as you so helpfully point out, they're not going to find it if they're not looking for it! I have come across lots of dodgy things, even in google searches (but perhaps my brain is more bent? i dunno)