I found the following story on @HistoryWeird's blog here. The original source was Dr. Maurice Laugier's "Du role de l'expertise medico-legale dans certains cas d'outrage public a la pudeur" in Annales d'hygiene Publique et Medecine Legal, 1878.
"In the 1870s Paris’ police and civic leaders railed against what they considered a significant problem: men soliciting sex from other men at public urinals. Consensual homosexuality was not illegal in France (it had been decriminalised during the Revolution) but public displays of homosexual behaviour were nevertheless prosecuted as 'offences against public decency'.
"Between 1870 and 1872 more than 100 men were arrested for loitering or acting suspiciously around street toilets in Paris. In 1876 police even found Count Eugene de Germiny, a conservative member of the city council, in a lavatory clinch with a young man named Pierre. After de Germiny’s arrest the concern about nefarious activities in public toilets reached fever pitch. One Paris physician, Maurice Laugier, attempted to penetrate [seriously? did they just use the word 'penetrate' in this article without at least a 'tee hee' written after it? Y'all over at @HistoryWeird are more mature than I am] the hysteria with an 1878 essay titled Du role de l’expertise médico-légale dans certains cas d’outrage public a la pudeur ('The role of forensic evidence in certain cases of outraging public decency'). Dr Laugier described several cases where men with verifiable medical conditions were unfairly dealt with by police, including one man:
'"…suffering from a urinary tract infection… who was observed and questioned by the police [for his] very prolonged stay in a urinal and the manoeuvres that he was exercising on his penis.'
"Men suspected of dubious activity in or around public toilets, wrote Laugier, should be questioned about their medical history and afforded a full medical examination before being charged or presented for trial."
Keep in mind, as well, that public urinals were VERY PUBLIC. This is what a public urinal in Paris looked like:
So we have the added issue of 'stage-fright', which I know affects people and can make them take quite a while to pee. Compounded on that is the idea that this is a very public place, so surely every man who isn't 'up to no good' would want to get out of there as soon as possible, therefore making men more nervous because if they take too long to pee, it will seem suspicious–which in turn gives them further 'stage-fright'.
For a more academic view of the whole situation (albeit in England rather than in France), I follow the author Lee Jackson (@VictorianLondon) on Twitter and know he's working on a book investigating things like public toilets and bath-houses. Keep your eyes peeled for this book–it should be really interesting when it comes out!