If, as Philip Larkin claimed, sex began in 1963, it appears to be fizzling to an end in the early decades of the 21st century. We British, never international avatars of sexual prowess, now seem to be living up to our billing: we are too tired, too stressed, or too drunk to screw. Surveys from both sides of the Atlantic show broadly the same patterns: we're having sex less often and when we do it is less satisfactory than ever before. We may be sexting, Tindering and OK Cupid-ing until our iPhones burn our palms, but when it comes to physical consummation, for many of us, sex has gone the same way as whist drives and tea dances. In the past two years, Suzie King, founder of celibate dating agency Platonic Partners, has seen a marked increase in visitors to her website (which began as a resource for the medically impotent). She puts this down to the common culprits: stress, depression and the mojo-sapping anxieties of the age of austerity. "We're in a rat race," she tells me. "That places an incredible strain on the psyche – there's no security any more. It used to be that one could work and earn enough to pay off the mortgage and then retire. These days, it's certainly not like that. This has led to 'pan-anhedonia' – low-grade depression all over the place. And one of the first things to go when we're depressed? Our libidos."

It may be that we're simply too enlightened. A recent spate of articles has laid the blame for the diminution in heterosexual activity on another, less obvious, source: feminism. In the New York Times, psychotherapist Lori Gottlieb dug out an old issue of the American Journal of Sociology and used a study within it – "Egalitarianism, Housework and Sexual Frequency in Marriage" – to support her claim that the waning of marital libidos was down to husbands doing too much ironing (and, implicitly, wives spending too much time in the office). Perhaps more interesting than the drop-off in erotic activity is the gleeful way that it is reported; a mixture of prurience and self-laceration driving these frantic swan songs for our sexual lives.

The stress of work and online life is having an effect in the bedroom, according to the study. Photograph: Thomas Northcut/Getty

The Observer's sex survey is the latest in a series of studies that point to a global decline in the frequency of, and satisfaction provided by, sexual intercourse. If you're in a heterosexual relationship and are over 35, then it's likely that you have sex less often than your parents did, and you're enjoying it less, too (an unsettling thought). At the same time, though, something new and interesting is happening behind the closed curtains of Britain's homes. For a significant minority – and this is true of both heterosexual and homosexual relationships – traditional genital intercourse is being replaced by other, less mainstream behaviours. Perhaps the real question we should be asking is what, these days, counts as sex?

Justin Hancock, a sex educator who runs the enormously popular Durex-sponsored Bishtraining website , speaks of a liberalisation of attitudes towards niche practices. "People have a much broader idea of what sex means, and the different range of sexual activity that they can take part in," he tells me. "There's now more of an understanding that sex ought to be mutually pleasurable and that there's a wide range of different ways to achieve that."

Indeed recent studies, most notably last year's Natsal Survey, the National Survey of Sexual Attitudes and Lifestyles, have shown a significant broadening in the palette of the nation's erotic activities: our sexual repertoire now runs from cuddling to oral sex, frottage, spanking and a dazzling list of fetishistic turn-ons from nappies to prosthetic limbs, with people having more partners and trying more practices than ever before. Rather than witnessing the death throes of sex, we may just be undergoing a radical re-evaluation of what is meant by the term. Cynthia Graham, an academic at the University of Southampton and editor of the Journal of Sex Research, agrees. "There's a broadening of people's views of what having sex means. Sex is no longer so closely geared towards reproduction. The rise of lesbianism, the rise of anal sex – these things aren't being discussed enough."

One piece of research Graham passes my way is the illuminating, if clumsily titled "Sexual Scripts Among Young Heterosexually Active Men and Women: Continuity and Change". The study, carried out by a group of North American academics, illustrates how, while outwardly acknowledging the roles prescribed by society for each gender – men as emotionally repressed and sexually voracious, women as monogamous and incurably romantic – the divergence from these normative stereotypes when couples are between the sheets is significant. People are finding their own ways to pleasure one another and old-fashioned, penetrative intercourse is no longer the sine qua non of sex. Graham tells me that couples are being more adventurous, and straying further from traditional sexual practices.

"There are a growing number of studies that look at people's widening definition of what constitutes sex. What they all show, and this is the same for whatever subgroup – gay or straight, across all territories – people are more likely to say that oral sex is sex, that any genital contact is sex. We've definitely been aware of an increase in other types of non-vaginal intercourse. There's been a real rise in heterosexual anal sex … the message that comes through is that there's been a genuine widening of people's repertoire."

We are working longer hours, and having less sex than our parents generation, the survey found. Photograph: Alamy

I meet sex blogger Vanessa Pelz-Sharpe in a darkly atmospheric Covent Garden restaurant, wanting to explore – in conversation, at least – some of the outer reaches of contemporary sexual experience. I've been reading Pelz-Sharpe's brilliant, often excruciating, musings on sex and sexuality on her site, Nightmares and Boners, for several years. Pelz-Sharpe was diagnosed with breast cancer early this year and now sits before me in a silk headscarf, her eyebrows pencilled on. (Nightmares and Boners has, for the moment, been put on hold, and Pelz-Sharpe is blogging instead at Sarcancerthon.) "For such a long time our thoughts about sex, our education about sex, have been focused on penis and vagina: man has an orgasm; maybe the woman does," she says. "There are so many things that misses – so many things you can do that aren't just the old in-out."

We move on to discuss the role of the internet in allowing people to embrace non-traditional sexual identities. She tells me about FetLife, a social network with more than three million members. "It's a bit like a fetish Facebook," Pelz-Sharpe says. "It allows people to not feel so alone, not to feel worried about the fact that they like whatever niche fetish it is. I mean, there are balloon-popping fetishes and farting. If you like gay sex, I imagine that, once you're old enough, you'll find other people like you. If you're into spanking, it's in enough soft-porn movies that you'll know you're not alone. If your fetish is farting, you probably do feel really alone and gross and dirty. Then you find all these other people online who're into the same thing, and who hold down jobs and aren't just sitting in a basement, and you think, 'Maybe I'm OK.'"

Pelz-Sharpe identifies herself as a queer polyamorist – terms that, even 10 years ago, would have carried very different meanings (or, perhaps to most people, none at all). This is something that strikes me again and again as I move through the world of non-traditional sexual practice: the importance of terminology and self-identification, the licence it grants people to feel themselves situated in a vibrant and inclusive landscape of sexual behaviour. Pelz-Sharpe came out as queer two years ago (she'd come out as bisexual aged 17). It's a term that casts a broad net, suggesting only a sexual identity outside of the mainstream.

"Queer meant you could be flexible, but also that you didn't think gender was a black-and-white construct," she tells me. "What I like about the term is that it's not a straight-up answer, it's the beginning of a conversation. I like the nebulous nature of it. It's OK to question these things, not to be penned in by a definition – straight or gay or whatever. I think of those people who live with a female partner for the first 20 years of their lives and then find a male partner for the next 20 years and look back and say, 'I was living a sham.' Why? If you really cared for that person, maybe forcing people into those boxes can be very damaging." The polyamory movement has gained considerable momentum in recent years. Pelz-Sharpe defines it as entering into relationships that are "ethically non-monogamous".

Definitions of normal sex are also changing, according to the study. Photograph: getty

"What 'poly' meant was that, if I was with a partner, it didn't mean that this was it for ever and ever. It also meant that I could have friendships and relationships that would be on the edge of acceptable in a monogamous relationship – things that might make your partner uncomfortable, like lying in bed with someone hugging them. When poly is good, it should mean that you don't own your partner; that you don't have any ownership over their behaviour or actions, that they are their own person."

Clearly some of the fall in overall sexual frequency is down to those who have chosen to abstain, or who identify as asexual. Here, again, is a community that has been given great licence by the advent of the internet. As Cynthia Graham tells me: "The asexual community is close-knit and has a real presence online. The internet has helped people to understand how much variability there is in people's sexuality, and total abstention is just one place on the continuum."

After a series of online conversations, I'm put in touch with the 22-year-old Australian Jo Qualmann, who blogs on asexuality at A Life Unexamined. "In terms of asexuality," she tells me, "I identify as an aromantic asexual." Asexuality encompasses a wide range of behaviours, from the merely disinclined to those who are repulsed by the idea of sex. "I probably fall more on the disinclination/indifference side of the continuum," Qualmann says. "I actually find sex really interesting from an intellectual point of view, and I've been interested in trying it out. But when my brain realises that I'd then need some sort of partner … it just kind of shuts down. The idea doesn't compute. I can't even start to imagine actually having sex with another person – male or female. And the same goes for kissing someone on the lips and making out, or even the idea of being someone's girlfriend. So to follow on from that, no, I haven't had any sexual experiences. I'm pretty happy to keep it that way 98% of the time."

I ask Qualmann how and when she first realised that she was asexual. "I was 19, in my first year of university," she says. "I'd spent the last five years or so identifying as a lesbian, and pretty openly as well. I realised when I was about 14 that I wasn't attracted to guys, and lesbianism just seemed like the default setting. I figured that I'd inevitably fall in love and have sex one day, but it wasn't really a priority at that time in my life. "But there I was, 19 and still not getting what dating and romance and attraction were all about. I started to feel that something was wrong with me, and that the whole situation was getting a bit pathetic. Eventually, Google pointed me towards asexuality. I thought I was pretty clued-up about sexuality – I had done a lot of queer activism in high school – but I had no idea asexuality even existed. It was like everything came tumbling into place all at once, exhilarating and terrifying at the same timeExhilarating because I finally felt like something described me perfectly, and terrified because it involved rewriting every assumption I'd grown up with about love and sex and humanity."

I suggest to Qualmann that it must be difficult to identify as asexual in a world in which we are constantly barraged by images of sex and told in magazines and newspapers (including this one) about new ways in which to bring ourselves off. "Being asexual in this sort of society can be incredibly alienating," she replies. "Because even though we've made a lot of progress in terms of the types of sex and relationships that are accepted, saying, 'no' or 'I'm not interested' or 'I don't get it' still doesn't seem to be a viable optionIn a way, the increased liberalisation of sex has actually pushed asexuality more into the fringes. But ultimately, I do think that we also need that liberalisation, because the next step is to say, 'so we've broken down a lot of old assumptions about sexuality – let's take that one step further still and start questioning some of the core assumptions we still hold'."

In The History of Sexuality, Michel Foucault talks about the scientia sexualis – the industry that has grown up around discourses of sexual behaviour. While there was something liberating about the freedom with which people began to talk about sex (Foucault was writing in the 1970s), there was also a sense that sexuality had entered the realm of science, where perversions were seen as problems to be treated and sex an act that might be medically altered; biologically enhanced. The industry of sexual confession, Foucault said, "set itself up as the supreme authority in matters of hygienic necessity … it claimed to ensure the physical vigour and the moral cleanliness of the social body; it promised to eliminate defective individuals, degenerate and bastardised populations."

We still find traces of this in current discourses around sexuality – the wish to identify deviations from normative behaviour and treat them as symptoms of some underlying sociocultural malaise. Instead, perhaps we should recognise that sex long ago stopped being about procreation, and most models of sexual activity that circulate at a societal level are woefully outdated when it comes to what we actually do with each other when the lights are out. From aromantic asexuality to anarchic polyamory, from fart fetishists to frotteurs, our sexual identities are as vivid and various as our selves, and this wild profusion is a thing to be celebrated.