Now that, one by one, European countries are shifting the law for allow for same-sex marriage – the spotlight now turns on Italy where mass demonstrations have taken place in support of the idea – it is  worth thinking about why homosexuality was criminalised in the first place.

We have spent so much time celebrating the decision in 1967 to repeal laws which did so, that we have perhaps forgotten to look a little further back to see how they came onto the statute book in the first place (sodomy had been a crime for some centuries before).

The story goes back to the Phoenix Park murders of 1882, when republican terrorists stabbed the Irish Secretary to death – accidentally, as it turned out: he happened to be walking with the intended victim.

The murders shocked the public on both sides of the Irish Sea, and to claw back the moral high ground, Irish Nationalist MPs launched a campaign to identify homosexuals in the Irish government, or part of the establishment in Dublin in some way – starting with the senior detective in charge of the Phoenix Park case, James Ellis French. The campaign led to huge torchlight processions and mass demonstrations, with bands, in many towns and cities of Ireland.

Most of the defendants were acquitted – the main issue at stake was whether it was physically possible to commit sodomy in a hansom cab.

The scandals barely ruffled feathers in London, except among campaigners linked to the Irish nationalist cause, or political friends of their parliamentary leader, Charles Stewart Parnell. Among these, the maverick Liberal MP Henry Labouchère was particularly frustrated that sodomy had been so difficult to prove.

So when the opportunity arose the following summer, as the Criminal Law Amendment Act – designed to raise the age of consent for women from 12 – crawled through Parliament, Labouchère seized his chance. His amendment was debated at night in a few minutes and only one MP queried whether it was relevant to the debate. But for the next eight decades, it put men – it only applied to men – in a perilous position if they loved their own gender. Up to 70,000 people were prosecuted under the amendment and many of them committed suicide, including Alan Turing.

The full story, and the link to my own great-great-grandfather – a prominent Dublin banker – who fled from Ireland at the height of the affair, is in my new book Scandal: How Homosexuality Became a Crime.

But the implication is that we have to be careful. Allegations of sexual abuse have to be investigated among politicians, of course. But when those allegations are brought for political reasons, it can have devastating consequences.

We need to remember this now that sexual allegations are being exchanged so freely in political circles. Once you stoke up a witch-hunt, the human consequences can remain for generations to come.

* David Boyle is a former Lib Dem parliamentary candidate and the author of Tickbox (Little, Brown). You can buy the book from Hive or Amazon.