Tourists used to be onlookers. Once upon a time we travelled with guidebook in hand, eyes wide open to the wonders of art and architecture, of new places and other people. Or that was the ideal. Now, to judge from this photograph of Italian tourists partying naked on the streets of Barcelona, the nature of travel has been reversed: the tourist is the sight, the wonder, the monster to behold.

It’s as if, in the age of the selfie, no one can stand to be a mere spectator. The centre of the show has to be me, me, me. The streakers’ parents may have travelled in tour parties led by a guide through La Sagrada Família or Las Ramblas but, at least to judge from this picture and other reports of casual mayhem at some resorts this summer, many people are now going abroad to make a spectacle of themselves.

At first glance, in this case, that show seems less offensive than it might be. Nudity in a public place is a defiance of social norms, but these young men at least have bodies that many might believe to be beautiful. It is possible to imagine a far less aesthetic display by young British males full of beer and fish and chips. But while Italian bad behaviour arguably looks more stylish – even at this naked extreme – than bad British behaviour, it is a revealing exposé of idiotic narcissism.

The men in the shop are right to be offended, and residents of this area of Barcelona plagued by mass, ostentatiously cheap tourism are right to protest that it is not only ruining their streets but wrecking the place for more refined (and moneyed) visitors. Cultured travel of a more traditional sort is good for cities, bringing money to restaurants and shops instead of just to booze merchants.

It’s not as if Barcelona lacks interest for more civilised visitors. The city of Picasso and Gaudi has loads to see. But looking has become a cliche: going on holiday to observe, let alone learn, is too self-denying for a generation obsessed with its own image as seen on a smartphone screen. “Forgetting to update your status – priceless”, said an advert at Athens airport when I arrived there a couple of weeks ago. Yeah, right – the punctuation of email alert beeps mingling with the cicadas on the slopes of the Acropolis suggested otherwise.

The fascinating thing about this glimpse of holidaymakers behaving badly is that it reveals something essential about the malaise of modern tourism – that loutishness is increased by social media, by the selfie age, by an era that turns everyone into a sub-Warholian self-regarding celebrity. You can’t just have fun any more. You have to be seen to be having fun, whether it’s by hiring a stretch limo for a night out, touring central London dressed as an animal, or taking all of your clothes off to shock the people of Barcelona.

This is – clearly – a generalisation. Lots of people go on holiday without taking their clothes off in public once. On the quiet Greek island I visited there was no sign of any of this summer’s widely reported drunken horrors. I got the impression some of the bar owners might have welcomed a bit of mayhem, if only to fill their half-empty terraces. Yet it is clearly hard for people to leave digital devices and the culture of self-display they promote behind. This picture reveals how that modern narcissism can turn tourism into a noxious plague.