Henry Badenhorst has certainly been a quiet revolutionary. As Gaydar, the website he co-founded 10 years ago, became the world's most successful online dating site, Badenhorst remained silent. The site has transformed the way people relate to each other on and offline, an influence reaching far beyond its original ambition of hooking up single gay men. But apart from Badenhorst's regular namechecks on gay power lists - he tends to vie for position alongside the likes of Elton John, Ian McKellen and Evan Davis - we know almost nothing about him.

He's had his reasons to keep quiet. Gaydar has hardly lacked for publicity - on the contrary, it has been a godsend to media scandal stories. When Lib Dem MP Mark Oaten was found to have engaged in a sex act with a rent boy "too disgusting to describe in a family newspaper" - as one paper noted - it was Gaydar that was implicated as the place where they'd met. When Labour MP Chris Bryant was found pictured on the internet sporting nothing but his pants, that was Gaydar, too. And when Boy George was convicted for falsely imprisoning a male escort earlier this year, it emerged that he had found the escort - you guessed it - on Gaydar. But through all the success and infamy, Badenhorst has remained publicly mute. Especially, since Gary Frisch, the co-founder of the site and his former life partner, died after jumping off his eighth-floor balcony in a drugs haze in early 2007.

Now Badenhorst is finally ready to speak, but not before a preliminary off-the-record chat in a central London hotel. I pass the test, it seems, because I'm invited to his office: Gaydar HQ. Not the chrome Soho penthouse one might expect, but a characterless 60s office block set back from a residential side street in Twickenham, southwest London, not far from the rugby ground. At first I struggle to hear him. He speaks in such a gentle voice that I have to lean in to make out what he's saying.

He starts at the beginning of the Gaydar story. "It was June 1999," he recalls. "We [he and Frisch] had a Dutch friend called Frank who was single and said: 'I need a boyfriend - can you help me?'" Frank didn't have time, it seems, to visit bars so, recalls Badenhorst, "we put him on Excite [a search engine], which had a dating section where you could upload a picture. But it took two weeks for him to get a response, so we said that we were sure we could create something specifically for the gay market." By November the site had launched.

Badenhorst and Frisch had moved to London from South Africa in 1997 to set up the IT firm QSoft, which provided revenue-management systems for airlines. They launched and ran Gaydar together - the innovation that set the site apart from Gay.com (the other destination for the date-hunting homosexual) and ensured its success was the creation of "profiles". These are simply a single web page for each user, an idea that's now standard on dating sites from Match.com to Mysinglefriend.com (neither of which are as popular as Gaydar, despite their larger target market).

Pictures were uploaded on to the profile pages, and information - basic, personal, sexual - could be written. There were sections for "stats" - height, weight, hair colour, as well as hobbies and interests, adult or otherwise, and a section on what members were looking for. The profile provided an opportunity to imprint some humanity on the anonymity of cyberspace. And to inform people as to whether or not, for instance, you still have your foreskin.

"Gaydar started as something we did on the side," says Badenhorst. "We didn't realise what we were creating, but then people started coming to the site. I placed some ads in [free gay magazine] Boyz, which drew in a few people, and slowly it grew. It certainly didn't take off from day one - the first year we had a several thousand, then the second year was 75,000 and then suddenly, in the third year, in 2001-02, there were more like 220,000."

Initially the site was targeted at those who already led an active gay life, going to bars and clubs. "I had a friend who helped me create the first ad. It said: '3am, the club was crap, I'm horny as hell, use your Gaydar.'" Ten years on, the success of the site has been blamed for gay bars and clubs going under. "Just an excuse," retorts Badenhorst. "If you have a good venue, people will not stay at home night in, night out." Now the majority of people who use Gaydar are not what in gay parlance would be called "scene queens". But the greatest transformation of all has been the way it has enabled those in rural areas - or countries where homosexuality is illegal or taboo - to connect with each other. "When I was a teenager," Badenhorst recalls, "I knew I was gay but I thought I was the only one; but these days boys go online and see there are plenty of gay men."

Plenty indeed. Five million people around the world subscribe, spending on average more than an hour on the site with each visit. Most pay a monthly £5 subscription, with the rest of the company's revenue coming from advertising. Now advertising is not difficult for Gaydar to come by, but in the early years "no one would come close," says Badenhorst. "We wouldn't even get as far as pitching - potential clients would just say they weren't interested." In 2004 that began to change. "Ford was the first. One of the people working on its campaigns was a Gaydar user!" American Express, BMW and Virgin followed.

Until then, they had even more fundamental problems with other companies. "The Royal Bank of Scotland closed our merchant account with just 24 hours' notice. They said someone had complained about it and so took the view that it was too much of a reputational risk." Now, of course, RBS has slightly bigger risks to its reputation than a few snaps of unclad gay men. But that wasn't all. "No hosting companies would deal with us either; they wouldn't touch anything with even remotely sexual content - but I'm sure the gay thing came into play. So we had to host the site ourselves - we had fibre-optic cables running into our house." (They initially ran the business out of their home in Twickenham.)

But by 2004, the success of the site could not be ignored by those eager to benefit from the pink pound. Also, by that stage the website had a new, "cleaner" sibling: GaydarRadio (which now has 1.6m listeners). "Suddenly here was a brand that people could associate with because it was nonsexual," says Badenhorst.

The site had already been very publicly associated with sleaziness. In 2003 the MP for Rhondda, Chris Bryant, could be found in his Y-fronts helpfully offering details of his requirements to anyone who chanced upon his profile. Then there was the Mark Oaten affair. "I think it's most unfortunate when these things happen, because it's just people going about their lives and it gets blown out of proportion," says Badenhorst. "It makes me angry because this [Gaydar] is for the gay community - who are you to judge them? If this was a straight site, would it be such an issue?"

Are there other politicians signed up to Gaydar?

"I'm sure there are. But I certainly don't search the database to see who's on there. If politicians want to use the site we'll do our damnedest to make sure their identity is protected."

The most recent Gaydar-related scandal involved Boy George. The singer was jailed in January for falsely imprisoning Norwegian escort Auden Carlsen after meeting him on Gaydar; he's since been released.

"George was always a great supporter of Gaydar, and in the early days he had a whole lot about it on his radio show, which we were always very grateful for." Presumably Badenhorst felt distinctly less grateful after the escort episode. "The Gaydar brand gets pulled into it," he agrees. "It's one thing using the site to meet people, but what you do thereafter is your problem. It was wrong what George did to that guy. It's not something you do to another human being."

But it is precisely the way in which gay men treat each other on Gaydar that has caused much of the controversy about the brand. Particularly surrounding the issue of "barebacking" - the practice of wanton, unsafe sex. Last year a More4 News report about how Gaydar has changed the lives of gay people concluded that Gaydar makes it easier to indulge an interest in barebacking. But Badenhorst is unrepentant. "People are going to have unsafe sex whether you tell them to or not."

But you allow people to advertise on their profiles that they are looking for condom-free sex - surely you could intervene?

"That would create more damage, because all you would do is push the whole barebacking thing underground. I would rather be in a situation where people are honest about their sexual practices, so whoever contacts them can make informed decisions about whether to meet up with that person."

Badenhorst also points to the work he and the site do to encourage safer sex. They have volunteers from the Terrence Higgins Trust in the chatrooms for any user to speak to whenever they want, and the company has a history of supporting other such charities, like Freedoms, a free condom-distribution company, and the National Aids Trust.

Another common concern is the extent to which Gaydar can encourage the baser aspects of male sexuality, objectifying potential mates into a sexual shopping list of attributes.

Badenhorst agrees - in part. "Online," he says, "it's easier for coupling to become a criteria of things you want." One of the more practical of the site's facilities is the "GPS" (Gaydar Positioning System), where you can locate all members who live within a mile radius. This can lead to your neighbourhood morphing into a veritable minefield of former conquests. One imagines. But on the more starkly dial-a-pizza-and-choose-your-toppings end is the "power search". Here, if you want to seek out a Middle Eastern 33-year-old with blue eyes who practises safe sex, is circumcised, has a stocky build, a hairy body but a bald head, who wears sporty clothes, is sexually passive, who smokes socially, drinks often but never takes drugs, who is a Sagittarius and has a small penis, then you can. It really is that specific.

But when I press Badenhorst further on this topic, a hilarious admission spills out. "Well, I don't always see how people interact on there," he says. "Because I don't use the system."

What? I splutter. You don't have your own profile on there? Badenhorst laughs.

"No... no... can you imagine?" he says.

But why not?

"I had a few bad experiences of people stalking me. When Gary died they got my name and then found my details from Companies House, so I would get weird things sent to me and people would phone my house in the middle of the night or leave abusive messages. I had to get lawyers involved."

So how does Badenhorst meet people?

"The old-fashioned way," he replies. "I go to bars."

For the first and only time in our conversation, Badenhorst clams up when I probe him on his current personal life. Have you been dating recently?

"Yes," he says, his eyes sparkling. Has that been a recent thing? "Absolutely." How does that feel? "Exciting." Do you feel any twinges of guilt? "Not any more," he replies, sadly.

Having worked relentlessly on the site for 10 years now, he seems somewhat fatigued by it all. "You see so many images [of nudity] that you start noticing things in the person's room - 'Ooh, look at the wallpaper!'" He is, however, proud of the many millions of connections - fleeting or otherwise - he has facilitated. "It's only when you meet people and they tell you how it's affected their lives that you go back and think: 'This is what I've done.'"

Badenhorst's success, however, has not been unerring. Last year, QSoft had to lay off a few editorial staff from GaydarNation, their offshoot entertainment website. In March, Badenhorst closed Profile, the Soho bar he co-owned. But, he insists, this was not for commercial reasons, and the bar will reopen under a different name. The lesbian arm of the site, GaydarGirls, while in no way a failure (325,000 users) has not caught on with anywhere near the same whoosh as Gaydar.

"The product is not right for them," he says, with Gerald Ratner-esque honesty. "The behaviour of gay men and lesbians is different."

Badenhorst was born and raised in suburban Johannesburg. His mother gave up her job as a theatre nurse when she married his father, who worked for the transport services. The second of four boys, young Henry was always different. "My mother must have known [that he was gay]. I never played with my older brother, or played rugby - I was always in the kitchen doing things. But I had a normal Afrikaans upbringing." Popular at school and never bullied, he instead had the Afrikaans church to contend with. "I had to go to a church that believes it's a sin to be gay and you'll burn in hell for it, so for years I struggled with why the church wouldn't accept me for who I was." Unresolved, he later left suburbia to move to Hillbrow - "the Soho of Johannesburg" - where he started attending a church "that was OK to be gay in". So OK, in fact, that "It turned out to be just a huge cruising ground - so that didn't last long."

Military service came at 18. "I had a great time," he says, laughing mischievously. Badenhorst was still not "out" to his parents. In fact, he says it was only "two or three years ago that I had an open conversation with my mother about it". Only then did his parents realise exactly what he did for a living.

In 1991, Badenhorst, who is now 42, met fellow South African Gary Frisch, two years his junior, in a "cruising ground... I always make jokes that he was the one-night stand that never went away." The laugh that follows is almost forced. On 10 February 2007, Frisch did finally go away. That Saturday afternoon he took ketamine, the animal tranquiliser and recreational drug, and jumped off the eighth-floor balcony of his Battersea home. The inquest recorded a verdict of "misadventure".

They hadn't been a couple in the last few months of Frisch's life. After 15 years together, and eight years running Gaydar, Frisch moved out. "We got to a point where we had become friends and because we worked together were seeing each other 24/7, so it was a mutual decision to break up. And Gary got to a point where he was tired of working the hours and wanted to have a bit of fun and live a bit, so he did things in that last six months before he died that he'd always wanted to do. He went white-water rafting in Zimbabwe, he went bungee jumping, he was recapturing his youth. He was going to bars and clubs and loved it. I couldn't understand it because I'd been there and done that."

And it was that recapturing of youth, that wanting to feel alive that led to his death? Badenhorst goes to say yes, but his voice cracks. "That was what I struggled with the most - if we hadn't parted, would the outcome have been different?"

How did he learn of Frisch's death?

"I got a call from the police that day... It was about 6pm that Saturday, and I was at home." The memory registers on his face like physical pain. What did the police say?

"That he had died; how he had died. And they said: 'I'll phone you back in 10 minutes. Phone somebody, get someone round and get yourself together.' I was alone at home."

So what did he do? Henry makes an exhalation from the back of his throat.

"You know, it is... it was the worst day of my life, the realisation that this had happened. I had shared a life with him for 15 years; I absolutely loved him. For minutes I would stop and think: 'Maybe it's not true, maybe I'm just imagining this,' and I think what I did was phone [friends and colleagues] Anna and Trevor, and they immediately came over."

The police questioned Badenhorst. "They wanted to be sure there was no reason it was anything other than an accident." But Badenhorst knew it was nothing more than that.

"I knew because I spoke to him 10 minutes before he died. He phoned me, we had a decent conversation. On the Friday I was quite worried about him because his frame of mind was not right. So he phoned me about 12 o'clock on the Saturday afternoon. He was busy getting ready, about to go shopping. I knew there was somebody there and I knew he was uncomfortable telling me who it was, and I didn't ask. But I got off the phone and thought: 'You know what? He's going to be OK.' They took the drugs prior to going shopping and so never made it out."

The man with Gary was Darren Morris, who later told the inquest that Frisch had stayed up all night on his own, and in the morning he found Frisch sitting on the floor with some magazines, saying: "Thank you, Lord; praise you, Lord." Then, according to Morris, Frisch put music on, started dancing and talking incoherently: "I came into the living room and I saw him standing on the balcony with his hands on the rail. He somersaulted over the top."

Stephen Ruddock, an estate agent, was outside when it happened, and revealed that Gary made a "Waheey" sound as he jumped. "It was a celebratory thing," said Ruddock. "I saw his body come into my line of sight. It arced in the air and hit the ground."

On the Monday morning the story was out. Speculation as to the cause of Frisch's death and his "mental wellbeing" began to grow. Was it an accident? Was it drugs? Depression? Badenhorst was besieged by reporters. "The media was camping outside my door, trying to get an interview, trying to find out if I was with Gary when it happened. I just said: 'I'm not going to talk to you.' It got so bad the police phoned a few papers and said: 'Please stop doing this.'"

Knowing that the press would run with the story on the Monday, Badenhorst was desperate to tell his employees of Gary's death before they read about it. So, first thing, he assembled the 70 employees at the offices and told them. "We did it in a group situation and made sure we had grief counsellors on hand for everyone. There was a lot of shock - some people cried uncontrollably, some people could talk about it, and some people are still uncomfortable with me talking about it."

Thousands of tributes poured in from gay men around the world whose lives had been changed for the better because of the website. But Badenhorst was busy taking care of the grimmest task of all - doing the ring-round, telling Gary's brother (his parents were dead) and friends. Then he had to clear out Frisch's flat. "That was the hardest thing, especially going back to the place where it happened."

At the funeral Henry was too distressed to speak. "I wrote something but somebody read it for me. I wasn't able to." At this, his eyes begin to glisten.

In the aftermath of the funeral and the inquest, there was something else going on. An old "friend" of Frisch's was suing Gaydar for a percentage of the company. "He wanted huge amounts of money. He claimed he supported Gary in the beginning, that there was a conversation between Gary and him that he would get a percentage."

Proceedings had begun before Frisch's death. "He was never somebody who could deal with stress very well. There was a lot of emotion going on with him dealing with issues around his mother's [recent] death, so adding litigation on top of that didn't exactly help."

Badenhorst concedes, however, that what led Frisch to taking ketamine - known for its hallucinatory effects - was "a combination of things": the stress coupled with his determination to recapture his youth.

Badenhorst's anger over the court case is palpable - a low, rumbling rage. "I was fighting for what Gary and I had built together. It meant I couldn't come to peace with Gary's death and I couldn't let it go, because every single day I was digging into the past to prepare for the case. I held our position, and he [the litigator] eventually went away. It went on for more than a year." They settled out of court.

What does Badenhorst miss most about Frisch? A long pause follows.

"His honesty. He would say what he thought, whether or not it would embarrass himself or anybody else. And his kindness. If anything bad ever happened to me it was amazing how much concern and compassion he would have. He always protected me." Indeed, even in his death, he did so, leaving Badenhorst the bulk of his estimated £6.5m estate.

The long-term implications have been profound. Even the website needed a year's work on it to unravel all the programming Frisch had done for it. "He hated documentation, so it was all in his head."

But only when that and the court case were over did Badenhorst begin to take stock. "I am finally looking at my life and realising what I've been through - at the time you're just on survival mode." Badenhorst says he wants to cut down his hours, and admits that now, with Frisch gone, his passion isn't what it was.

"If you had asked me before he died if I would ever retire, I would have said: 'Absolutely not.' I always wanted to be in control, and it was something that we were passionate about together. So you start viewing your life differently and start thinking: 'Maybe there is more to life than just working on Gaydar.'"