The Los Angeles Times called it an "unapologetic crowd pleaser of a movie" and the film website Rotten Tomatoes has awarded it a stonking 94 per cent rating, but American censors didn't have quite the same warm glow when viewing the hit British drama Pride.

Starring Imelda Staunton and Bill Nighy, the life-affirming story of lesbian and gay activists helping Welsh families in the 1984 miners' strike has delighted critics on both side of the Atlantic. But the Motion Picture Association of America ruled that the film was unfit for 16-year-olds unless they were in the company of a parent or adult guardian. The draconian US rating – for a film that is rated 15 in the UK – has caused anger among filmmakers and gay activists. "It is outrageous, knee-jerk homophobia," says veteran campaigner Peter Tatchell. "There's no significant sex or violence in Pride to justify strong ratings. The American classification board seems to automatically view any film with even the mildest gay content as unfit for people under 17."

But the MPAA's stance is not a surprise. Love is Strange, an endearing French-American drama about two gay Manhattan men in a long-term relationship, was given an R rating in August, supposedly on the grounds of "strong language". The film, starring Alfred Molina and John Lithgow, was rated 15 by the British Board of Film Classification.

A year ago, the director Darren Stein complained that his gay-themed film GBF had been rated "R for Sexual References" despite "not having a single F-bomb, hint of nudity or violence in the film".

In an angry post on Facebook, he suggested: "Perhaps the ratings box should more accurately read 'For Homosexual References' or Too Many Scenes of Gay Teens Kissing'." Again, the BBFC rated it 15.

It is not as if gay themes are a new thing in American film. The first recorded reference was in The Dickson Experimental Sound Film, which was made in 1895 and showed two men dancing in a tight clinch. The scene is said to have "shocked audiences with its subversion of conventional male behaviour". And it is more than 20 years since Tom Hanks struck an historic blow for equality with his Oscar-winning performance as Andrew Beckett in Philadelphia, the first mainstream film to deal with HIV and Aids. That film was rated PG-13 in 1993.

The MPAA has not given reasons why Pride is considered inappropriate for unaccompanied 16-year-olds.

On its website, the BBFC explains its 15 rating of Stephen Beresford's film as being based on "occasional strong language" and some scenes with sexual references. One shows "men in a gay club wearing 'bondage' clothing", while in another scene "women roar with laughter as they find a dildo and some gay porn magazines in a bedroom". The BBFC adds: "The images in the magazines include some strong sexualised nudity, but the images appear only briefly and in a very clearly comic context."

Film industry sources noted that the BBFC carries out surveys of British public attitudes every four to five years. The MPAA, meanwhile, must apply a single rating across a larger country, where moral values are more diverse. "The Bible Belt is very different from New York City," says one source. "People think that the UK and US systems are like for like but they are really not at all."