Through some freak accident of PR, I was invited to an event organised by Tinder. If you’re over 40 or have become prematurely married, you might not know what Tinder is. It’s the mobile-phone app that facilitates courtship by allowing people to signal their interest in other users within a certain radius — you can set it to just a mile, if you’re in a real hurry to ‘connect’. It’s the modern human version of mating calls and frog croaks.

A million Londoners are said to use it. But Tinder is now under threat. Trendy dating apps such as Happn or Hinge, which present themselves as a bit less nakedly Darwinian, are growing in popularity. In response, Tinder is launching Tinder Plus, and the venue for the launch is a boutique gym called One Rebel. Tinder Plus lets you find out a little bit more about people, lets you change your mind and retrieve people from the dustbin, and also allows you to trawl foreign cities in advance of travel. Also, the new app is trying to preserve the youthful aura of Tinder by charging older users (that’s over-28s) nearly four times as much as younger users. It’s because younger people have less money, says Tinder, but a cynic might say that it’s in their financial interest to ensure there is plenty of young flesh in play.

One Rebel is in the City, right by the Gherkin. In the window a video is playing of people exercising under flashing lights in an intense, focused, urgent, almost angry way, with a slight hint that they’re also rebelling against a stifling political order like in The Hunger Games. ‘High Intensity Re-shape Workout’ flashes up. I have agreed to join one of these sessions, which has a special Tinder theme.

In the locker room I talk to a nice American guy, thirtyish, who works for Twitter (not Tinder). I mention Tinder — does he use it? He gives a slight laugh, as if to say who doesn’t. ‘In New York I use it for dating, but when I’m travelling I use it more for just meeting people to have a drink with.’ Oh, so men as well as women? ‘No, no, it’s, er, still structured around meeting women, but, er, I use it more in a social way rather than a dating way?’ It seems to me that this young man, when he’s travelling, is more interested in making a friend to hang out with for an hour or two, to escape the lonely hotel room. Of course he’d like this to be a lovely young woman, but he’s not obsessively hunting for that. So Tinder can be used in a non-sexual way, but only unofficially as it were: its basic narrative remains ‘We’re all on the prowl.’ It helps people to make social connections — at the cost, perhaps, of sexualising social life.

The workout is slightly embarrassing: there’s a lot of working in pairs, swapping partners, like a party game (I feign a pulled muscle to avoid the worst of it). There’s some mild Butlins banter about getting to know each other, not being shy. Our instructor is a nice East European woman with a body like Madonna’s. (I’m reminded of Guy Ritchie’s rude comment that sex with her was like cuddling a bit of gristle.) When we swap partners we are told to ‘swipe right’, in imitation of Tinder.

Afterwards I meet Rebecca, who does Tinder’s publicity. She’s pretty and posh, like a young Samantha Cameron, and looks me in the eye a lot and stands quite close, so that her excited gesticulating always risks touching me. I tell her about my locker-room conversation and she’s very excited. ‘Yes! It’s about social connection, not just finding a date. You can use it however suits your life. It emulates your life.’ She also says it’s ‘morals-dependent’, meaning it’s up to you whether you seek quick thrills or more subtle and slow-burning interaction with someone. Isn’t it rather brutal, the way one so swiftly judges a person by their appearance? ‘But that just reflects life — it’s human nature, you walk into a bar and assess if you like the look of who you see, you go yes, no, yes, no, which is like swiping right, left. We live in a very image-based society, Tinder reflects that. And yes, it’s about the photos people put up, but you can say a lot in those photos about your interests, what sort of person you are. And also you can check out what mutual friends you have, so that’s about more than looks. If you see someone’s got mutual friends who’ve got great businesses or whatever, then great — synergy! So yeah, it’s exciting. What? You look unsure about something.’

Then I talk to a young woman who works at the gym. She’s been on Tinder for a year or two, she says, but has only been on a couple of dates in that time. In both cases, she says, they had mutual friends on Facebook, so she could get a character reference before meeting up. She wouldn’t meet up with a total stranger, she says, though some of her friends do. How did the dates go, I ask a bit pruriently. ‘Not great, but it was an interesting experience.’ Soon I chat to -another young woman who used the app for a while, before she met her present boyfriend. ‘I never went on a single date. It was just a laugh really — me and my friends would look at it at the pub, see who was liking us.’

My inquiries over the next few days produce a rather old-fashioned picture: men on the prowl, women scorning their advances and giggling with friends about it. One man tells me: ‘Just about every guy I know that uses it doesn’t even look at the screen, but just keeps swiping right until they exhaust all the people within the radius and age range they’ve set. As soon as more became available they right-swipe those too. Women, however, are much more picky, looking in great detail at every picture posted and only right-swiping those guys they really ‘fancy’, knowing that pretty much every guy has yes-swiped them anyway so they’ll get a match. But for lots of women it’s more of a titillation and an ego-boost than a practical -dating tool.’

It’s hard to say whether Tinder encourages casual sex. Most men would say, if only. What it does seem to encourage is the idea that casual sex is more available than ever, and that if you’re not getting it then you’re a mug. In other words it ramps up the pressure yet further, the culture-sized peer-pressure to venerate the thrilling pursuit of sex.

Larkin once sighed at the ‘printed directions of sex’. Technology gives the rule of sex awesome new authority, and a totalitarian smell.