I’m not quite sure what the difference is between keeping the kids’ books in a specific section, which is what I see, and having an “adults’ only” section, like you’re talking about. Are you saying librarians need to check ids to let people browse the stacks? Let’s not argue about checking out materials because that’s a different matter, you’re refering to “sections” in the library.
And I think parents should supervise their children’s library access. If Mom scans through books BEFORE the kid goes to check them out, then we have nothing to throw fits about.
R-rated and NC-17 and X films are offlimits to minors. Yet they can rent the DVDs (and you will find some X dvds, some may have been rerated NC-17 on submission for re-rating after their original 70s release, some X tapes of things like the Passolini videotapes remain in some libraries).
Since all of those are “pulled” by the librarian, it’s possible to ask for ID (or have restrictions on library cards to check them out).
Bookstores may have a “top shelf” for some materials, but they don’t prohibit 18+ materials. But they sure aren’t dumb enough to sell such materials to minors because they know that they face arrest if they do.
However, this thread is NOT about "checking books out of the library."