Here's the first lesson on joyless side of sex

London's streets were dangerous places for innocent young men in uniform to judge from the world's oldest sex education film, which has just been revived for posterity by the British Film Institute.

Filmed during the First World War, the silent footage features a young Canadian soldier called Dick – a name that seemingly had no unfortunate connotations back then – who is on leave in London where nicely dressed young women approach him, one after another. Dick, the caption says, is "tempted", but luckily, as he is about to meet his doom, a Canadian officer taps him on the arm. "Do you realise, young man, the risks you run in association with that woman?" he asks, silently.Cut away to a seedy hotel room where another Canadian soldier has not been so well advised. Though he and the fallen woman he has met are fully dressed, a ruffled bed is evidence of the risk to which he has exposed himself. While his back is turned, the shameless woman goes through his wallet and stuffs a wedge of notes into her bra.

Back to Dick, now on a guided tour of a hospital ward where men are being treated for venereal disease, where he is shown horribly swollen legs and claw-like hands. "Rotted legs and hands", the caption reads. The message is very clear: there is no such thing as safe sex for a soldier overseas so, laddie, keep your mind on your pure young girlfriend at home and say no.

The Canadian film, Whatsoever a Man Soweth, is the oldest of 16 classic sex lectures on celluloid that the BFI has put together on a two-DVD set, documenting more than half a century of the advice, warnings and euphemisms used to ward off disgrace among the young.

Fast forward 56 years, to 1973 and 'Ave You Got a Male Assistant Please Miss?, and the film-makers seem relaxed about the idea of a man and a woman together in the sack. However, just as the couple are about to get down to business, a disembodied voice says: "Hold it, hold it a minute. Did you know that in 1969 there were 10,000 illegal abortions?"

They did not, but the man – who incidentally has hair down to his shoulder blades – gets the message, whips on his trousers and set off at a run to find a chemist, where he is served by a male with even more hair. Over the decades, evidently, the emphasis has switched to avoiding accidental pregnancy. "There is no need to make an abortion of it," is the final caption.

But even in the relative enlightenment of the early 1970s, casual sex was a cause for condemnation, for young women if not for young men. This point is emphasised by the 1973 film Don't be Like Brenda, Brenda being a foolish young woman who ends up pregnant, shunned by everyone around her, waiting in vain for the phone call from her ex-lover that never comes.

Not all the contributions are so grim. The 1932 production The Mystery of Marriage, which bills itself as a "frank" explanation of sexual attraction, is a joyous romp through the marital arrangements of plants, insects, birds, mammals, and humans. It tells you that butterflies are "frivolous" parents, but that most birds are good parents. There is a vague hint that there is something these couples do that produces children which, when humans do it too, seems to involve a walk in the fresh air, a kiss near a haystack, followed by a meeting with an estate agent at which she says: "Then the lino will be laid free?" And her husband says: "And I only have to pay £10?" That was as explicit as a film could be, in our grandparents' day.

Katy McGahan, who produced the collection The Joy of Sex Education for the BFI, said: "The films document the various approaches educators have taken in disseminating important information about sex to the populace across 60 years.

"Today, with teenage pregnancy rates and reported cases of sexually-transmitted disease soaring, sex education remains high on the political agenda. While most people accept that formal sex education is necessary, the question over who should teach it, how it should be taught, what information it should impart and what moral and social values it should inform remains ever contentious."