When you sit on the loo in Ed Houben's tiny bathroom, there's a postcard at eye level that says "Welcome to Maastricht". It's decorated with dozens of smiling tadpole-shaped creatures homing in on the words with cheerful intent. It's a little touch to make his visitors smile; after all, most of them are here for Ed's sperm.

Houben has been donating sperm for more than 10 years now, first at the local sperm bank and then, after reaching the clinic's legal limit, privately via the internet. Most of his donating is done in his neat, modest flat on an estate on the outskirts of the Dutch town. The T-shirt he's wearing, which declares "Anytime, anyplace, anywhere, anyway", is actually a bit misleading. "In the old days I would gladly travel, and my colleagues covered for me if I was late to work," he says. "But my job at Maastricht's tourism office has changed and I have to be around much more. Now I ask people to come to me."

And they do, from all over the world. Houben has biological offspring in Australia, Israel, Canada, Cyprus, Germany and Luxembourg, as well as at home in the Netherlands. His current tally of donor-conceived children stands at an eye-watering 62, with the 63rd on the way. It is by no means a world record – Houben once watched an episode of Oprah about a man who has fathered 200, a number he says he'll never catch. But he has been called Europe's most prolific sperm donor, and he's happy to accept the title until someone has the, well, balls to challenge him.

In his late thirties, Houben is on what you'd term the cuddly side, and he'd be the first to deprecate his looks, although the digital photo frame that's scrolling through his brood – some fair, some dark, some hairless, puckered babies – reveals an unexpectedly attractive crop of kids. He's genial, bright and witty on the subject of his pastime; but what his recipients value most of all is his commitment.

This week he's seeing three women in four days, two from Switzerland and one from Germany. Luckily the old scientific thinking that abstinence improves the quality of your sperm has recently been disproved (Houben wasn't surprised – he once got a woman pregnant two hours after failing with another). He takes his sperm count seriously – there's a recent test result lying on his desk, measuring everything from volume to pH and motility – and he's extremely well read on the latest fertility research; he also takes folic acid and fish oil.

As it's illegal to sell your sperm in Europe, donation is a vocation rather than a career. Houben got the calling in 1999, after witnessing the trials and heartbreak of childless friends undergoing fertility treatment. Donor numbers were declining dramatically in the Netherlands (from 900 in 1990 to 200 in 2002) and those affected by the shortage, particularly lesbian couples, turned to the internet. With his state-sanctioned quota fulfilled, and much more to give, Houben started placing ads on the websites and online forums that were springing up. He scored with his very first attempt; the day we meet, that child is celebrating her seventh birthday.

Houben's first UK baby is expected later this year to a couple who, having heard about him, bought their first computer specifically to track him down. Last weekend he had more than 20 of the children he's helped create at his home for his once-yearly get-together (he had a T-shirt for that occasion too: "Veni, Vidi, Vici"). "I always felt the child should have a right to know where it comes from," he says, and adds that he hopes that his openness on the subject will help to tackle the stigma surrounding sperm donation. "Before I became a sperm donor, my own first thought was of an old guy in a long trench coat. I want to take that image away and raise the level of discussion."

Many European countries, including the UK, are suffering shortages in their sperm banks, limiting treatment through public health services. For those who find themselves at the bottom of long waiting lists, or denied treatment altogether – particularly single women, gay couples and those starting a second family – the options are costly private treatment, or the do-it-yourself approach. And there is no doubt that the online sperm donation scene is growing. One popular website, Free Sperm Donations Worldwide (FSDW), claims to have 1,100 UK donors; if that's true, it's outstripping the NHS three to one.

Co-parentmatch.com was set up four years ago by a UK couple. Jenny Kearns, 33, and her partner were looking for a donor themselves, and after two years of research they pioneered an online membership community which works exactly like a dating website, allowing donors and recipients to search and browse each other's profiles until they find what they're looking for. It now has 10,400 members, roughly 40% of whom are donors.

"We weren't interested in going to a sperm bank because we wanted our child to know the father as they were growing up," explains Kearns. "On our site you can state what level of involvement you're looking for from a donor – if any at all – and the site will match you up with the right people." Members pay £9.95 a month to send and receive messages, and those hunting for donors are, says Kearns, an even split of lesbians and heterosexuals. "Most of it's to do with the costs and processes of fertility clinics, a lot of couples want to bypass that system. There are strict regulations and long waiting lists for sperm in the UK, but we never have a shortage on our site."

The website has the smiling, professional look of a Harley Street address offering upmarket dentistry, and Kearns says it is carefully monitored. But the fact is that, outside of the health system, sperm donation is an unregulated, underground activity and sexually transmitted diseases, unknown medical histories, and clandestine meetings with complete strangers all pose genuine dangers. Co-parentmatch recommends that women always request to see medical test results before considering a donor, but "ultimately it's down to members to do checks," says Kearns. "We're not a clinic."

And it is not only risky for women. Unless they donate to a licensed clinic, men aren't exempt from parental and financial responsibility for any begetting they do. In 2007, Andy Bathie, a fireman who fathered two children to a lesbian couple, lost a test case against the Child Support Agency, which had demanded he pay maintenance when the mothers split up. For serial donors, however altruistic their motives might be, it is a potentially bankrupting situation.

It begs the question: why do it? And if it's hard enough to motivate more than 300 people in the UK to register as sperm donors each year, who are the men offering themselves out, week after week, for nothing more than tea and biscuits?

To try to find an answer I head online and discover John, a regular donor who went private after he reached his NHS quota and is, like Houben, in considerable demand. He's paranoid about his identity and rather bossy on the phone, and when he agrees to meet me at a nondescript location, in between appointments, I'm expecting to encounter the seedy, sinister side of the sperm trade. I'm pretty surprised when he turns up on a bicycle and orders a green tea.

It turns out John holds down a career in IT while answering SOSs from ovulating women all over the country, often travelling hundreds of miles a week because he hates to turn anyone down. Once he's committed to helping someone, he'll keep trying until the job's done. He keeps records, doesn't spread his seed too close to home, and he charges nothing more than the AA standard mileage rate of 40p a mile. I tell him he sounds like a superhero. "You have to be quite flexible really," he says modestly, "and sometimes you find yourself running around all week. But I knew it was going to have a profound effect on my life."

John started donating in his early 20s after a serious accident made it hard for him to have a normal relationship – "I thought that donation would be rewarding where other parts of my life weren't," he tells me. Now, he says, his duties mean a family of his own is unlikely, but it's a sacrifice he's willing to make. "'Donating often makes dating someone new seem more trouble than it's worth. This isn't like being a blood donor. This is a lifestyle."

I can't see any special reason why John wouldn't find a partner if he wanted one. He's in his thirties, decent looking, intelligent and articulate, if a tad jumpy, and a little obsessed with his subject. But he seems to have well reasoned, altruistic motives for donating privately – he believes a woman should be able to meet and choose the donor who suits her best, rather than have the decision made for her at a clinic – and a genuine passion for what he does. To be honest, if that passion was anything other than impregnating women, it might be quite charming.

John offers several methods with varying levels of discretion. He has, in the past, produced for sperm delivery services, in which a courier comes to his house and bikes the freshly harvested product straight to the waiting customer. He will help out a single lady with the syringe if required; he will meet husbands so that they can compare hair and eye colour; alternatively, he can be thoroughly invisible on the job. One of his clients tells me that she leaves a room prepared and the back door open, and has never actually seen him ("He's very professional, he leaves everything in a nice and neat order").

And like many of the online donors, he is also an experienced practitioner in "natural insemination" – in other words, sex. Sperm donor forums bristle with terminology, but the most regularly used are AI and NI, and there is considerable debate in the community as to whether offering NI makes you a hero, an opportunist or a pervert. FSDW instantly bans any donors offering NI; Co-parentmatch says it recommends against donors who insist on NI only. But John says it remains an important option for those who want to hurry the process along "if they feel their fertility might be limited, or they want to keep the costs down".

Houben, who also offers NI, agrees. "From my own experience, statistically NI is faster," he says, and he has records to back up his claim. "I take off my hat to the guys who only do AI, but if people are coming all the way from Italy they don't want to be trying for three years." He only started offering NI when a couple specifically requested it – and he got a further shock when he discovered that the boyfriend expected to be present at the insemination. Fortunately his sense of duty prevailed.

Houben points out that while it's easy to accuse donors of looking for cheap sex, most of the women he sleeps with aren't people he would choose as sexual partners. "This isn't Heidi Klum coming round and saying: 'Let's do it'," he says. "It's genuine people who I would never want to hurt. I have a good old-fashioned Catholic guilt feeling and I would be a candidate for therapy if I did this for the wrong reasons."

Houben and John, however, represent the higher end of the service; they are honourable guildsmen of the sperm fraternity. Both are extremely committed and are choosy about who they help (Houben has certain health requirements of recipients – non-smoking, light drinkers, not overweight – and also looks for some kind of sympathetic connection). But they are aware of a less discriminating, and less palatable, marketplace. John says plenty of donors use it as a money spinner, charging ludicrous, unsubstantiated expenses; Houben remembers an early German donor website he discovered populated with "men who had clearly typed 'sperm' in Google looking for porn". "It was gruesome," he grimaces. "It was basically the Wild West."

An idle 10 minutes browsing Tadpole Forum gives you an idea of just how unpleasant and unsafe sperm donation can be. It's a UK site, the very first result on a Google search for "free sperm donation", a place where users have handles like "Sir Lancelot" and "Exotic DNA", and big-talking guys boast about their IQ, their looks and their virility. The seemingly unmoderated message boards are riddled with gossip, innuendo and the occasional libel: users accuse each other of having multiple identities, falsifying references, sexual assault, and even – in one long-running thread which has unfolded like a compulsive soap opera plot – of donating while knowingly infertile.

John calls Tadpole "the dodgy backroom bar of the community – the kind of place you wouldn't want people to have to go" and argues that its existence demonstrates how desperate some women are for help. "Sites like that are a direct result of the government's failure to create a working regime," he says angrily. "The current guidelines act to create an enormous and totally unnecessary shortage of donor sperm."

The figures certainly a hit a new low in 2004, when the Human Fertilisation and Embryology Authority announced that donors were to lose their right to anonymity, and donor-conceived children would be allowed to request their biological father's details at the age of 18. And while registration numbers have rallied since – thanks in part to recruitment drives – Dr Allan Pacey, senior lecturer in andrology at Sheffield University, points out that the UK is now having to import 20% of its sperm from abroad. "The number of sperm donation treatment cycles performed has gone down year on year, yet we know that there are waiting lists, sometimes of years. It's at odds with the message the HFEA puts out, and I'd argue there's a problem."

Some fertility campaigners say they would like to see better compensation for donors; others say that the system is too localised, and a central sperm bank is needed. Houben and John both argue that current limits on donations per individual are arbitrary and unsupported by science. The general consensus is that, in the meantime, unregulated websites are filling a void. "The fact that there are alternatives is very concerning," says Susan Seenan of the Infertility Network. "If there wasn't a shortage, people wouldn't be forced to look at that option."

To see matters from another perspective, I head to the West Country to meet two of John's happy customers. Laura Bennett, 35, and her partner Mandy Wedgwood, are playing with their six-month-old son Izak on the grass outside their home. Bennett, a doctor, knew that their local NHS trust would only provide donor sperm to couples with proven infertility – in other words, they had to have a go themselves before they could get help. "If you're a same-sex couple," says Bennett, "you're in a bit of a predicament."

The couple met some "very weird people" ("There are men out there who think they can take over the world by procreation") before finding John, and it took Bennett seven attempts to get pregnant, at hotels of differing provenance along the M4 corridor. "I am so relieved Izak wasn't conceived at the service station at Newport Pagnell," she says. "It had brown Artex on the walls. It felt like we were exchanging crack cocaine instead of beginning a new life…" They are, however, delighted with the result, and have kept a scrapbook of their experiences – including a piece of that brown Artex – to show him where he came from.

John has never met Izak, or any of his biological children, and says he has no reason to think they'll play any part in his life. He admits it leaves him soul-searching about the future: "If you choose this, what do you fill the gap with when you're older?" It's a question that's occurred to Houben, too. How long he'll continue adding to the world's population he's not sure, but he can't see a reason to stop yet. Unlike John, Houben does have a girlfriend – someone who originally came to him looking to get pregnant – and she is happy for him to continue. "But," says Houben with a smile, "we might have a family together. And then all bets are off."

To find licensed fertility clinics, or to register as a sperm donor, go to hfea.gov.uk