In between half-hearted attempts to flex himself into the shape of a pretzel on his yoga mat, Ogden, the star of Inappropriate Yoga Guy, the current cult online fitness video, makes lecherous advances towards Kimberly, a lithe and buxom classmate, "omms" too loudly and brags about the retreats he has attended in Nepal. During one asana, the headband-wearing yogi grabs Kimberly's hips to adjust her pelvic tilt. While everyone else in the studio is searching for spiritual enlightenment, Ogden, it seems, is simply searching for a girlfriend. His hilarious antics appear to have struck a cord - the comedy sketch has attracted nearly 2 million viewers on YouTube over the past four months.

To anyone who has attended more than a handful of yoga classes, the reason is obvious: at one time or another, most have come across their own Ogden. Among the committed yogis who limber up with inner focus, there usually lurks one with a different intent, namely to slip in a chat-up line between poses and use the class as a speed-dating forum.

I once attended a weekend yoga workshop where I was accosted, more or less from the outset, by one of the few men taking part. He would position his mat next to mine, offer to get my hot water and lemon when we stopped for a break, and indiscreetly dive across the room to be my partner when attempting a move that required one. At first, it was mildly irritating, but by the second day I was finding his behaviour deeply intrusive. So, apparently, was the teacher, who had noticed his oddness towards me and asked him to leave.

I am not alone. A quick survey of friends and colleagues who practice yoga revealed that 12 out of 16 have had similar encounters. One recalls how a man who positioned his mat too close to her own would touch her body during floor twists until she complained to the instructor. Another was repeatedly hassled by a male class member who wanted to wine and dine her. "I wasn't interested, but he would accost me after every session," she says. "I found it very offensive because my yoga class is my little sanctuary and he was invading that."

Most dedicated yogis agree that there are unwritten rules: no talking, no looking around, no grunting. And definitely no gawking or pick-up lines.

According to Pierre Bibby, chief executive of the British Wheel of Yoga, 95% of people who attend classes are women. "But more and more men are taking it up and more men are becoming yoga teachers," he says. "There is a definite shift."

It could be argued that yoga has always had an image that veers somewhat closer to the sexual than other fitness activities. All those tantric connotations, visions of extreme suppleness being practised in flimsy, loose- fitting clothing, and a focus on emotional contentment, are bound to get some people steamed up. Its reputation is not helped by the fact that websites such as yogaromance.com and yogapassions.com, designed to introduce single yogis looking for love, are springing up all over the internet, and classes such as "social flow" yoga (after which students go for cocktails) are already up and running.

Jonathan Sattin, a director of the leading Triyoga chain in London, says that "yoga and its teachers are perceived in a particular way by clients and we have to set strict guidelines and codes of conduct to make sure there are no inappropriate relationships".

At Triyoga, as with many other establishments, teachers are strictly forbidden from having relationships with clients. But sexual interest in the gym isn't restricted to the harassment of female yoga students and too much heavy breathing in the yoga zone.

Away from the tantric, the wider gym has always been a mecca for singletons in search of more than a smaller waistband - and there is nothing necessarily wrong with that. Tracey Cox, relationship expert and author of More Hot Sex, says that, if you like working out, gyms are the obvious place to meet someone with like-minded interests. An advantage, she says, is that you get to see far more than you normally would on a first date - Lycra is the opposite end of the scale to speed-dating in the dark. "Exercise is a great way to meet people," she says. "Your judgment isn't affected by alcohol, and you're seeing the person as they are, stripped back to the basics."

Cox adds that "it's easier to make friends in a 'non-date' setting, like the gym, where a smile and 'hello' is less likely to be seen as a pick-up line". But for those who need a helping hand, this is a market set to boom in the next couple of years.

Meanwhile, in Australia a new iniative has just been launched - outdoor classes combining exercies and dating in a "flirtatious four-week fitness programme." Founder Erica French says the classes comprise 16 "sporty singles" - eight of each gender - looking for love. And in America, gym-bunnies can sign up for websites dedicated to helping fitness fanatics pair up, with names such as buffsingles.com or fitkiss.com. Sites such as dateactive.com or dateactive.com offer a similar service for lonesome exercisers in the UK.

Some though, need no encouragement to develop a full-blown crush in a steamy, sweaty workout environment, even if their passion is unrequited. My friend Lisa, 35, from London, admits that she became hooked on attending classes, not because of the yoga itself, but to swoon at the man who was teaching them. "A friend of mine told be about this gorgeous yoga instructor who was teaching at a top London studio and asked me to go along with her. His classes were jam-packed with women and he had this wonderfully soothing voice. When he came round to tweak our thighs into position or adjust our pelvic alignment I would go bright red with embarrassment. It was so obvious I had a crush on him, but then so did 90% of women in that class."

It is something with which Richard Ferguson, who runs the RJ Yoga studio in Berkshire, identifies. "Flirting with a teacher is something that happens all the time," he says. "It is partly because, as a teacher, you do engage with people very closely and you do have to pay some attention to their emotional needs," he adds. "I have never crossed that line, but I know many, many yoga teachers who have succumbed to the flattery and attention".

· This article was amended on Monday October 22 2007. This article about men doing yoga was illustrated with a photograph of a T'ai Chi class. The picture has been removed.