I’m thinking for me it was Carnal Knowlege (1971) starring Jack Nicholson, Art Garfunkel, Candace Bergen, and Ann-Margaret.

And here’s some history of the MPAA rating system:

Replacement of Hays Code

The Hays Code, in place since 1930, was deemed by the MPAA to be no longer appropriate for the current film environment. The Code was revised in 1966 to include the “SMA” (Suggested for Mature Audiences) advisory. The MPAA’s film ratings system was instituted on November 1, 1968 by the industry in order to avoid censorship by local jurisdictions.

The original movie ratings (used from 1968 to 1970) were:

• G: General audiences – all ages admitted
• M: Mature audiences – parental guidance suggested, but all ages admitted
• R: Restricted – children under 17 not admitted without an accompanying parent or adult guardian
• X: No one under 17 admitted

This content classification system originally was to have three ratings with the intention of allowing parents to take their children to any film they choose. However, the National Association of Theater Owners urged the creation of an adults only category, fearful of possible legal problems in local jurisdictions. The “X” rating was not an MPAA trademark: any producer not submitting a movie for MPAA rating could self-apply the “X” rating (or any other symbol or description that was not an MPAA trademark).

From M to PG

The M rating was changed because parents were confused as to whether “M”-rated films or “R”-rated films had more intense content. This led to the “GP” rating in January 1970^.

The ratings used from 1970 to 1972, were:

• Rated G: All ages admitted. General audiences.
• Rated GP: General audiences, parental guidance suggested.
• Rated R: Restricted. Under 17 requires accompanying parent or adult guardian.
• Rated X: No one under 17 admitted.

Also in 1970 the ages of viewers admitted to R- and X-rated movies was raised from 16 to 17. However, the age on the X rating varied per the jurisdiction.

By 1972, parents perceived the GP rating as not indicative of a film’s true content. In 1971, the MPAA added content advisories such as: Contains material not generally suitable for pre-teenagers. In February 1972 the MPAA replaced the GP rating with the new PG rating.

The ratings used from 1972 to 1983 were:

• Rated G: General Audiences—All ages admitted.
• Rated PG: Parental Guidance Suggested—Some material may not be suitable for pre-teenagers.
• Rated R: Restricted—Under 17 requires accompanying parent or adult guardian.
• Rated X: No one under 17 admitted.

By late 1978, the PG rating was reworded. The word pre-teenagers was replaced with children.

The PG-13 rating is adopted

In 1984, explicit violence and gore in the films Indiana Jones and the Temple of Doom and Gremlins caused an uproar among parents over their PG rating.^[11] Their complaints led Hollywood figure Steven Spielberg, director of Temple of Doom and producer of Gremlins, to suggest a new rating to MPAA president Jack Valenti. Spielberg’s suggestion was for an intermediate rating of PG-13 or PG-14. On conferring with cinema owners, Valenti and the MPAA on July 1, 1984, introduced the PG-13 rating indicating that some material may be inappropriate for children under 13.

The first two films distributed with a PG-13 rating were Dreamscape and Red Dawn (1984) although The Flamingo Kid (1984) was the first film so rated.

The ratings used from 1984 to 1985 were:

• Rated G: General Audiences — All ages admitted
• Rated PG: Parental Guidance Suggested — Some material may not be suitable for children.
• Rated PG-13: Parents are strongly cautioned to give special guidance for attendance of children under 13 – Some material may be inappropriate for young children
• Rated R: Restricted — Under 17 requires accompanying parent or adult guardian
• Rated X: No one under 17 admitted

From 1985 to 1990 the PG-13 rating was described thus: Parents strongly cautioned — Some material may be inappropriate for children under 13

X replaced by NC-17

In the rating system’s early years, X-rated movies, such as Midnight Cowboy (1969), Fritz the Cat (1972), and Last Tango in Paris (1973) were understood to be non-pornographic films with adult content. However, pornographic films – if rated at all – sometimes self-imposed the non-copyrighted X rating. Thus, the X rating (along with the hyperbolic “XXX”) soon became a synonym for pornography in American mainstream culture.

This association led many newspapers and television stations to refuse advertisements for X-rated movies and some cinema owners refused to exhibit them. Such policies led to the distributors’ compromise with George Romero‘s horror film Dawn of the Dead (1978). Participating NATO cinema owners agreed to enforce the audience restriction rating, but the letter X would not appear in advertising. Instead, the following content warning advisory message was displayed: “There is no explicit sex in this picture; however, there are scenes of violence, which may be considered shocking. No one under 17 will be admitted.”

In 1989, two critically acclaimed art films, The Cook, the Thief, His Wife & Her Lover and Henry: Portrait of a Serial Killer, were released featuring very strong adult content. Neither was approved for an R rating, thus limiting their commercial distribution. Director David Lynch suggested an RR rating for such adult-oriented films.

On September 27, 1990, the MPAA introduced the rating NC-17 (“No Children Under 17 Admitted”) as its official rating for adult-oriented films bearing the MPAA seal. Henry & June was the first film to receive the NC-17 rating.

The ratings used from 1990–1996 were:

• Rated G: General Audiences — All ages admitted
• Rated PG: Parental Guidance Suggested — Some material may not be suitable for children
• Rated PG-13: Parents Strongly Cautioned — Some material may be inappropriate for children under 13
• Rated R: Restricted — Under 17 requires accompanying parent or adult guardian
• Rated NC-17: No children under 17 admitted

In 1995, the NC-17 rating age limit was clarified by rewording it from “No Children Under 17 Admitted” to “No One 17 And Under Admitted”.

What was your first R-rated movie?