The change in attitudes towards sexuality in recent times has been extraordinary. For instance, anyone middle-aged or older can remember when, both legally and socially, homosexuals were treated as pariahs. This year, homosexual marriage became legal in England, Scotland and Wales. Though Orthodox Judaism regards gay unions as unacceptable, Masorti rabbis have now joined Liberal and Reform in conducting gay marriage ceremonies. Even the Pope has asked "who am I to judge homosexuals?" and has said they have "gifts and qualities to offer to the Christian community".

Although even this relatively mild statement was last month rejected by a meeting of cardinals, who were prepared to say no more than that discrimination against gay people was "to be avoided". True beyond question is that the freedom that gay and transgender people feel to announce and express their sexuality has removed a considerable cause of human misery.

Crucial and pioneering in the progress towards this point was the scientific and campaigning work of Magnus Hirschfeld, celebrated alongside many facets of sexual history in a major exhibition running for the next 10 months at London's Wellcome Collection, the scholarly medical research establishment that bills its exhibitions as "the free destination for the incurably curious".

There are 200 objects spanning art, erotica, science and lust down the ages. Sigmund Freud, source of so much of our sexual understanding and misunderstanding, will be a major presence but the exhibition's title, The Institute of Sexology, is a homage not to Freud but to Hirschfeld and the Institute of Sexual Science he set up in Berlin in 1919.

Born in 1868 in Kolberg on Germany's Baltic coast (now in Poland), into a secular Jewish family, son of a famously public-spirited doctor, he and his two older brothers all became doctors (in 1900, 16 per cent of German doctors were Jewish).

A founding father of the study of sex and sexual behaviour as a science known as sexology, Magnus Hirschfeld's commitment to tireless empirical research was a precursor of the work Alfred Kinsey and Masters and Johnson have done in modern times, research that has revealed what had always remained secret - the truth of our varied behaviour as sexual beings.

This secretiveness was, of course, far truer of homosexuals. In Germany, and indeed in England at the end of the 19th century, theirs was a hidden world. Hirschfeld, a homosexual, was deeply affected by reports of Oscar Wilde's imprisonment in 1895, which preceded a ferocious witch-hunt of homosexuals in Germany. He was a liberal, a trenchant public intellectual and a lifelong campaigner against Paragraph 175 of the civil code that made homosexual acts a crime. The Nazis further toughened the law and, as we know, many thousands of homosexual men were imprisoned and died in concentration camps.

As far as the Nazis were concerned, Hirschfeld and all his works were everything they hated. On May 6 1933, with Hirschfeld living in exile, Nazi students seized his institute and set fire to 20,000 books, 35,000 photographs and over 40,000 case studies. He saw the conflagration on a newsreel in a cinema in Paris.

He had written attacking the Germans' "race science". He was outraged at being discriminated against for being a Jew just as he was outraged at being labelled and discriminated against as a homosexual.

He wrote: "I am a German, a German citizen like Hindenburg, like Bismarck and the former Kaiser. An honest German born in Germany to German parents. Because my parents identified with the Mosaic faith, I am marked with the Mosaic stigma."

Freud's response to antisemitism was rather different. In 1925, he wrote: "My language is German. My culture, my attainments are German. I consider myself German intellectually, until I noticed the growth of antisemitic prejudice in Germany and German Austria. Since then, I prefer to call myself a Jew."

Hirschfeld was against stigmatisation of any sort for reason of gender as well as race. Homosexuality had been categorised as a pathological condition, an illness. He endeavoured to show through detailed research what an enormous range of sexual activities went on and that, rather than just the two clearly defined genders, there were more nuances and variations and pursuits than most people had ever dreamed of. Transvestitism and transgender were things he logged and understood. For him, these were not abnormalities, they were, with homosexuality, "the third sex". Freud was interested in Hirschfeld's research and learnt a lot from it.

The other Jewish-born star in the sexological firmament is Wilhelm Reich, whose wild and fairly deranged life ended, at the age of 60, in a US penitentiary. Starting off as a young protégé of Freud in Vienna, he became an apostle of sexual permissiveness and, as time went on and his theories got wilder, was viewed as a genius, a fraud or a crackpot. He certainly viewed himself as a genius and, moving on from Freud's concept of libido, he developed his theory of the orgasm as more or less the secret of life, his discovery of "orgone" as a source of human energy and his orgone accumulators (a box you sat inside to acquire the power) became fashionable with the sort of people attracted to life's weird, wild and probably spurious possibilities - Norman Mailer, for example, owned several. "Reich's notion that the orgasm in a certain sense was the essence of the character, gave me much food for thought over the years" he wrote.

In 1954, Reich fell foul of the US Food And Drug Administration seeking to ban the transportation and sale of orgone accumulators. In 1956, he was sent to prison and six tons of his books and papers were incinerated. His supporters denounced it as an appalling act of censorship. It was proof once again, if proof were needed, that sexology always has the power to get people fired up.

The Institute of Sexology exhibition at the Wellcome Collection, London NW1 2BE, opens on November 20