On a sagging blue sofa in a harshly-lit room, a topless woman sits next to her partner, his hands inside her pants as she writhes in pleasure. Their foreplay has lasted around four hours, from nipple play to blowjobs, the occasional ping of a retro slot machine going off intermittently in the background. After a while she climbs on top of him, and with a mop of dark hair covering all but the cusp of a smile on her face, leans forward and speaks so directly, she could only be speaking to you.

You, and 563 others.

At any one time, 5000 people are watching something similar on Chaturbate, a website where people broadcast themselves having sex for money (the portmanteau, in case it was lost on anyone, comes from "the act of masturbating whilst chatting online").

The site works like this: viewers choose a 'room' they wish to enter where they can watch anonymously for free, or pay to see something more explicit. Virtual tokens, which have been bought in advance, are used to 'tip' broadcasters so that they perform certain acts, either for the benefit of everyone or (for more tokens) during a private show. Games are played to sustain interest, such as offering surprises for random tip amounts -"20 tokens! Here's a pussy flash!" - or more elaborate set-ups involving naked Twister and spinning Wheels of Fortune.

"Our relationship has only become stronger since we started doing this"

Scanning the windows into the various rooms on the Chaturbate homepage, you can see bodies of every shape and size, like the contents of a Bakerloo Line carriage has been asked to disembark and strip off. Thousands of performers described as "chubby", "big boobs" or "hairy" sit alongside more conventionally attractive young men and women. Watching the wall of images flicker and body parts contort between positions is strangely mesmerising.

The chance to watch a couple having sex live, rather than actors performing cartoonish orgasms and comically acrobatic maneuvers, has been a game-changer for pornography in the digital age. Many professional porn stars have duly flocked to the camming world as a side project to earn extra money. But the community on Chaturbate is different. Here, genuine couples who found love first are getting in on the action. The question I want to answer is why any couple who have never been sex workers before would decide to take the plunge. What would it do to your relationship? And is it really all just about money?

Rebecca and Andy are an American couple who write an anonymous sex blog. Last year after months of browsing shows together they decided to take the plunge and start having sex on their webcam. "I can't remember which one of us suggested it, but there was enthusiasm from both of us," Rebecca says. "It definitely started as a sexual adventure - initially, we just wanted to have fun on camera while strangers watched us."

Today, they command hundreds of dollars for a short session. Balancing tips and intrigue is a fine line. "If you give the audience what they want to see for free, then they automatically realise that they won't have to tip you," Rebecca explains.

"This is why sometimes we just cam for fun with no pressure."

"On average we make $200 per hour"

How can putting the most intimate part of your life on display for the whole world to see feel care-free? Or to pose the question many might ask: how do you relax knowing your neighbour / boss / Mum could see it?

The answer is that Chaturbate allows you to block geo-locations so cities or whole countries are unable to access your broadcasts. Restrict it to places you have no ties to, and in theory you're pretty safe. That said, your identity is still vulnerable and there have been instances of viewers attempting to blackmail broadcasters with recorded images and videos. Whilst Rebecca and Andy are aware of the risks, they take a fairly measured mindset on the matter. "If you're getting naked on the Internet, then you need to think that everyone will see you being naked on the Internet," they say.

Last year the New York Times reported figures from industry insiders and analysts estimating that leading camming attract up to 25 million unique views a month and traffic monitoring website place Chaturbate's unique visitors at just under 18 million a month. The money they generate is hundreds of millions of dollars at least, and as Newsweek reported "possibly upwards of $2 billion annually." Amazon's website ranking system Alexa rates Chaturbate as the 150th most popular website in the world and the 144th within the US.

For a website, the economic rewards for what is essentially user generated content are surprisingly substantial. It costs viewers $10.99 for 100 tokens, and broadcasters take home $5 for every 100 tokens spent on them (1 token equalling 5 cents). It's an exchange rate which works in Chaturbate's favour but also allows the cammers themselves to clock up serious earnings. Professional models, the website's top earners, often share stories of earning online - "More than $1500 by working 3 hours a day from home in just 15 days!"- but couples can do very well out of it too.

Shirley Lara is Chaturbate's Chief Operating Officer, and has been with the company since its launch in 2011. She attributes the appeal of camming to the viewer being able to essentially make custom-made porn with a small budget. "Instead of having mass-produced content, the idea is the consumer buys into a specific person," she tells me on the phone from California. "It allows the user to be the casting agent and director, instead of having generic videos." In a world of entertainment services lining up to let us customise our viewing habits, it is no surprise that porn got there first.

"You could see it as something akin to swinging."

As for the broadcasters, Lara says they follow two distinct patterns: "those who use it as their career and sole income and those who might be a little short on rent and / or jump on it once or twice a week for fun."

Megan and Mason, a couple from Canada, tell me how for them camming is both a lucrative project and a way to express themselves sexually. "On average we make $200 per hour and we typically broadcast 4-6 hours a day," Megan says. "Camming has brought out the best in our relationship," she continues. "Being exhibitionists ourselves, performing for others is exciting and when the camera is off we're still intimate with each other."

Megan is not the only performer to claim it has improved her relationship. "I truly believe our relationship has only become stronger since we started doing this," another anonymous user says.

PJ Patella-Rey is a former couple cammer who is now writing a thesis for his PhD around the sex camming industry and aiming to get a holistic picture of the craze which has erupted in the last five or six years.

He argues that it is a natural extension of the early internet 'live camming' craze where people would turn on their webcams for hours at a time and have people watch them go about their day. "This is a new generation of cammers," he tells me, "who are instead are turning the camera on for specific periods of time and using it for sexual exploration."

PJ believes there's a range of reasons why a couple would decide to broadcast their sex lives. "I think all of the highly visible couples are doing it primarily for money, if you look at the first page you see very professional and calculated couples who have really figured out how to do this and turned it into a job and are really good at it.

"But then you go to the later pages and you start to see something different. For some it's a thrill like exhibitionism, and there's also couples who are interested in exploring new areas of their sexuality with the encouragement of others, but want to do it together."

He thinks of it as sex clubs for the always-on age. "You could see it as something akin to swinging. Cultures where couples have explored their sexuality with other people or other couples have existed for decades. They are looking for a sort of intimacy by interacting with other people.

"After all," he continues, "if they weren't seeking interaction, these couples would just watch recorded porn. There's a lot of positive feedback in it for the people who aren't just doing it for the money, so affirmation is something else viewers have to offer."

The idea that part of the appeal is having your attractiveness affirmed by hundreds of people watching you naked is emphatically confirmed by the couples I speak to: "The feeling of importance and the attention is a definite turn on" one pair say.

PJ believes this sense of connection is essential. "The feedback loop that exists between viewer and performer is what they are seeking, being part of something bigger than themselves. I think that is why lots of these other sex sub-cultures have existed for so long, people were looking for that kind of group sexual energy and dynamic in a way," he says.

Many of the traditional barriers of sexual experimentation are wiped away with Chaturbate. Can't get to a sex club? Don't have a babysitter to go swinging? Don't want to actually watch your wife be physical with someone else? Here is virtual sexual playground you can shut down at any moment. Instead of causing jealous feelings, PJ believes it can in fact dispel resentment in relationships - for example, one person may have bisexual urges and be able to gratify them by performing sexual acts for members of the same sex online, rather than doing anything in the physical world which could prove a 'step too far' for the relationship.

"It was a cool thing to do but but we totally lost control."

But despite the positive feedback from the site's most successful couples, the concerning aspects of the industry are there if you look for them. In 2016 the National Crime Agency claimed webcam blackmail cases were up more than 50% from the previous year. Though most of these cases relate to viewers being tricked with fake videos of women enticing them to strip off, blackmail, threats and extortion are a growing problem for broadcasters too. The majority of shows on Chaturbate are being recorded, mainly to be resold or uploaded to free porn websites. In some cases they are also used to blackmail broadcasters with threats to show footage to their friends, family or colleagues.

In an interview with Mic, a cam performer named Eliza Jane spoke of how she knew "several peers who quit the industry after trolls published their personal information online, sometimes even physically showing up and stalking the performer in person." Jane went on to recount an occasion where a viewer said to her, "I know it's you," and then wrote her real name. This mirrors many similar stories where broadcasters are threatened with exposing videos to unaware family members if they don't pay them off.

For PJ and his partner, their camming experience didn't have a happy ending. "We stopped a few months ago having been doing it for several years," he says. "Piracy is now such an issue that virtually every show is being recorded by bots. I believe we are at the tipping point where it will start to completely undermine the communities online and destroy what camming is, turning it into something that looks more like mainstream porn."

Camming world offers a representation of sex with a level of diversity that is genuinely exciting...

You might think people who put it all out there on the internet would have no cause to care about it being reproduced elsewhere. But PJ argues the piracy issue is bigger than that.

"Part of the appeal of camming is the ephemerality of it, that it disappears instead of piling up all of these digital footprints. In a four hour cam show there will be some bad angles you don't want out there. Even for people who don't particularly care about being naked on the internet, you do care about unflattering naked photos of you. Moreover, it cheapens your product because it competes with you. Someone is giving away what you're selling, so why would anyone pay for it?"

PJ argues that cam piracy should be prosecuted under revenge porn laws rather than being solely an issue of copyright, and believes platforms like Chaturbate could do more to protect the broadcasters.

"It was a cool thing to do but but we totally lost control of our content," he says.

"You will see a winnowing of all the people doing it for sexual fun in an amateur way. In the same way we have YouTube stars like Pewdiepie, we'll just have camstars who are micro-celebrities, famous enough that they're able to make six figures and can justify spending several hours a week filing takedown notices as an operating cost."

If PJ is right and sex camming is being slowly packaged up and commercialised, its soul gutted like so many exciting corners of the internet before it, then there is perhaps cause for lament. Though the top tier of the site runs like a series of small-scale businesses, the rest is a representation of sex with a level of diversity that is genuinely exciting, where differences aren't fetishised but presented plainly and celebrated. Here's a small village of the internet that answers many of the concerns people have about porn causing unrealistic expectations of sex and bodies, a place where real couples show you what real sex looks like. In "Vagina: A New Biography (2012), feminist critic Naomi Wolf argued that "porn takes the sexiness - that is, the wildness - out of sex." It would be a stretch to claim Chaturbate offers a moral solution to the porn epidemic, but it is reclaiming something of that lost wildness of sex.

Back in the bright room with the blue sofa, the session is drawing to a close as the brunette girl says goodbye to the hundreds of strangers still watching around the world. She expertly twists her faded cotton underwear up around her ankles and with a flourish pulls them back over her hips, a grin spread from one flushed cheek to the other. Running through viewers usernames like a roll call, she announces the time they'll be back online and repeats it, basking in something between a post-sex glow and having hit the jackpot.