The MPAA reserved their distinction of ‘X’ for only the most salty-mouthed, violent, or sexually libertine content, making it an instant marker of controversy when deployed for a new release.
In 1990, they introduced the rating of NC-17 for films with such explicit content that they’d be unsuitable for any children under seventeen, regardless of adult accompaniment.
In the sweet spot between those two points, the X reigned supreme.
In 1968, the Motion Picture Association of America replaced their nuance-free, prohibitive Production Code (a.k.a. the Hays Code, the reason couples in old movies had to sleep in separate beds) with a system of alpha-numerical ratings denoting mature material or its absence.