The ALA’s stance is don’t let your child go to the library unescorted until age 18.
Their position on giving such materials to minors would not hold to any retailer.
Even if some “questionable” material did get through to kids, theaters won’t sell tickets to R rated movies to minors (typically they buy another ticket and try to sneak over), video stores typically won’t rent R or “the unrated version” of films to minors, and stores won’t sell Playboy or “Adults Only” tagged comix to kids. The public library will.