Nick Meadowcroft-Lunn has a girlfriend, whom he has been seeing for three years. Jezz Palmer has a girlfriend, too, and they have been together for five. You might assume therefore that Nick is straight and Jezz is gay; or, if not, that both must be bisexual. But you would be wrong.

“I always describe my sexuality as: ‘If you’ve got nice hair and pretty eyes, I’m down for it,’” explains Jezz, a 26-year-old editor working in historical publishing. “It’s not that gender doesn’t matter, because it can be important, but it’s a bit of an afterthought. It’s just like: ‘Oh, hello.’” For a while, she wasn’t sure what to call this, but eight years ago she settled on “pansexual” as the closest word. “It took me a while to figure it out. [The TV series] Torchwood was about the only thing I’d heard of. I was talking about maybe being pansexual and someone said: ‘Oh, like Captain Jack in Torchwood.’”

Nick, a 22-year-old physics and philosophy masters student at the University of York, initially thought he was bisexual as a teenager, but also now feels “pansexual” better fits his view that attraction isn’t really about gender. “I just find characteristics generally about people attractive. Pan is simply easier to understand, and much closer to the truth for me. It’s not specific to any gender.” He often explains it, he says, by talking about height: a bi person might find tall guys attractive, and short girls. But he tends to fancy tall people, regardless of whether they are male or female.

Last year, “pansexual” briefly became the online dictionary Merriam-Webster’s most searched word of the day after the singer Janelle Monáe defined herself as a pansexual and “queer-ass motherfucker”. The Panic at the Disco frontman Brendan Urie and the singer Miley Cyrus both also identify as pan, with Urie explaining that, to him, it means: “I really don’t care … If a person is great, then a person is great. I just like good people, if your heart’s in the right place.” The singer Demi Lovato, meanwhile, identifies as “sexually fluid”, or “having a shifting gender preference”, while other labels for being neither exclusively straight nor gay include “heteroflexible” and “questioning”.

For bisexual activists who have long felt erased from the picture, many of these new identities can sound suspiciously like elaborate ways to avoid the word “bisexual”. But Meg-John Barker, psychology lecturer and author of The Psychology of Sex, argues that, while “bisexual” is a useful and widely understood umbrella term for being attracted to more than either gender, labels such as “pansexual” do capture a specific sense that fancying someone isn’t just about gender. And if all this seems confusing, the all-purpose “queer” is increasingly used to mean anything other than plain-vanilla 100% straight, a visibly expanding category.

Facebook Twitter Pinterest Pansexual performer Demi Lovato (left) live in Lisbon. Photograph: Pedro Gomes/Redferns

When YouGov asked people to place themselves on a sliding scale where zero equals exclusively straight and six equals exclusively gay, more than a quarter of Britons polled identified as something other than 100% heterosexual. But strikingly, 54% of people aged 18 to 24 did. That arguably makes them the most sexually liberated, least socially repressed group of adults in British history.

Baby boomers saw homosexuality decriminalised, if not destigmatised. Their children grew up with Brookside’s celebrated lesbian kiss and the scrapping of Section 28. But it is their grandchildren who have grown up taking the idea of gay rights almost for granted. “The working assumption is that’s because we have progressed as a society in the last 30 years. We’ve become much more accepting and that’s allowed people to explore their sexuality,” says Paul Twycock of the LGBT rights group Stonewall.

And yet, for all that, heterosexuality is hardly dead yet. According to the Office for National Statistics, 93.2% of Britons still call themselves heterosexual, although that figure is down slightly from 94.4% in 2012. So how did YouGov get its headline-grabbing figures? It changed the question, which turns out to change the answer significantly.

Being attracted to more than one gender is becoming a majority position. But it's taking a long time to catch up to that Meg-John Barker

It is well over half a century since Alfred Kinsey, who was himself bisexual, published his conclusion: “Males do not represent two discrete populations, heterosexual and homosexual … The world is not to be divided into sheep and goats.” His successors are still arguing over whether the godfather of research into human sexuality was broadly right to describe it as a sliding scale with numerous stopping points along the way, or whether that is overly simplistic. But in popularising the idea that same-sex attraction was far more common than acknowledged, Kinsey’s work was a landmark moment for gay rights nonetheless.

When YouGov asked its respondents whether they were straight, gay, bisexual or something else, 89% identified as heterosexual and 6% as gay. But when asked to place themselves on the Kinsey scale, that fell to 72% straight and 4% gay. The more choices people are given, the more shades of grey they acknowledge. But does that mean heterosexuality is genuinely rarer than we think, or is sexuality more multifaceted than was previously accepted?

According to one US study, half of male college students and eight out of 10 female ones have fantasised about someone of the same sex. (Evidence is divided on whether women are more sexually fluid than men or just more willing to admit it.) More than a quarter of British 25- to 39-year-olds told YouGov they had had some kind of same-sex experience. But Generation Z are not necessarily having more adventurous sex than anyone else; they are more inclined to what might be called a “never say never” approach, with a quarter of those identifying as straight saying they couldn’t rule out a gay relationship if the right person came along.

“This suggests that being attracted to more than one gender is becoming a majority, not a minority, position,” says Barker. “But wider culture is taking a long time to catch up to that fact, still tending to assume that people are either straight or gay, and presenting non-binary attraction as confused, a phase, or somehow suspicious.” The gradual easing of those assumptions, however, has implications for more than one generation.

Andrea Hewitt has known since her schooldays that she was attracted to girls. But growing up in the US south in the 1970s, she didn’t dare think too hard about what that meant. “I didn’t really know any gay people until I was an adult. I didn’t understand a lot of the feelings I was having, so I put them on a shelf,” she recalls. “It just wasn’t an option. Nobody spoke of it.”

So, she duly got married and had two children; when that marriage broke down, she married again. It was only after her older daughter left for college that she finally plucked up courage to come out as lesbian and ask for a divorce.

Hewitt’s children and her wider family were supportive, but it was, she says, an isolating time. “I Googled ‘coming out’, but it was all geared towards teenagers coming out to their parents, and here I was a 40-year-old woman with two kids. I truly thought I was the only person who had ever done this.” It was only when she started her blog, A Late Life Lesbian Story, that she realised she was very far from alone.

Bestselling US author Elizabeth Gilbert and her late partner Rayya Elias. Photograph: Instagram/Elizabeth Gilbert

Two years ago, the author Elizabeth Gilbert revealed she had left her husband Jose Nunes – the man she described travelling halfway round the world to meet in her bestseller Eat Pray Love – for a female friend, Rayya Elias. The British retail expert Mary Portas famously fell in love with the fashion writer Melanie Rickey after an amicable divorce from the father of her two children. Hewitt now runs a Facebook group for women coming out in later life with more than 1,100 members worldwide; while some identify as lesbians, others prefer not to define their sexuality or swear they were straight until the moment they fell for a woman. But one common thread, says Hewitt, is having parked their own lives on a back burner while they were raising children. “I’d say a lot of the people in my group have a very similar personality type. We’re mothers, we’re fixers, we’re problem-solvers; we want to focus on everything but ourselves. It isn’t until you have time to do some self-reflection that you go: ‘Wait a minute, what about me?’”

Hewitt, who now lives in Nashville, Tennessee, with her partner Rachel, says she cannot be sure that if she had been born two decades later she would have identified as lesbian from the start. But while some of her Facebook group wish they had had the courage to do so years earlier, she cautions against assuming that the marriages of women who come out later must have been a sham all along. “You can only know what you know when you know it. You can’t go back and judge your past self on thoughts you didn’t have.”

Changing social attitudes are clearly enabling some older people to explore feelings repressed for decades. But coming out in middle age does not necessarily imply a life spent in the closet, according to Barker, who points to the US psychologist Lisa Diamond’s landmark study following 79 non-heterosexual women for 10 years. The women originally identified as either lesbian, bisexual or preferring not to put a label on their sexuality. Over time, two-thirds of their sexual identities shifted, and a third changed more than once; overall the most common identity adopted was “unlabelled”, and more women moved towards identifying as bi or unlabelled than away from it.

Yet, as Hewitt points out, the idea that sexuality can change across the course of a life is threatening for some. “If you allow for the possibility that people can change their sexuality, what’s to say your wife couldn’t do that, or you couldn’t?” Some of the later-life lesbians she knows were asked when they were going to “change back” to being straight, while one of her own friends suggested that perhaps she hadn’t just met the right man yet.

And if it is difficult for seemingly straight people to come out as bi, then it is perhaps even more controversial for gay people to do so. If sexuality really is fluid, then it might logically be expected to flow both ways; yet in practice it is not always easy for members of a historically oppressed group to admit to sleeping with the perceived enemy.

Part of the reason bisexuality and sexual fluidity are so erased is because they’re seen as muddying the water Meg-John Barker

The idea that sexual identity is set in stone has been useful in some ways to the gay community, especially in tackling the offensive idea that homosexuality might somehow be “cured”. Parents struggling to deal with their children coming out are often encouraged to accept that sexual preference is just something we are all born with, as immutable as race or age and just as deserving of protection from discrimination. So, what if it isn’t as fixed as we thought?

In the US, Diamond’s work has been used by campaigners against same-sex marriage, who argue that it shows some gay people can change their minds – even though Diamond has stressed the changes she saw were involuntary and sometimes against the women’s wishes. Meanwhile, even pointing out that having visible bi role models in public life can help teenagers to come to terms with their own bisexuality risks being twisted into an argument that kids are only choosing it because it is fashionable.

But the pressure to argue for gay rights on the grounds of fixed identities has, Barker argues, led to some inconvenient truths being swept under the carpet. “Part of the reason bisexuality and sexual fluidity are so erased and rejected is because they’re seen as muddying the water.” When Antoni Porowski of the TV show Queer Eye, which involves a panel of gay men making over a generally hapless straight one, came out as sexually fluid, he was accused on social media of being a traitor and a fake, despite having been with his boyfriend for seven years.

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Kate Harrad is a bi activist and editor of Purple Prose: Bisexuality in Britain, a collection of essays exploring all forms of bisexuality. One of the recurring themes in the book is, she says, people describing going to an LGB group or bar for the first time “and being rejected by the gay and lesbian people they met because they ‘weren’t really queer’ or ‘hadn’t made a choice yet’ or because they were seen as innately faithful and untrustworthy. Imagine finally getting up the courage to go to a place you think will accept you and instead experiencing hostility or scorn, or disbelief that your sexuality even exists. It’s no wonder bi people have worse mental health than any other orientation.”

Bisexual people are also less likely than gay ones to be out at work, which Harrad argues is not surprising: “Bisexuality is heavily associated with explicit sexuality, for a lot of people even more than gayness is. So people feel entitled to ask weirdly intrusive questions, like how many people you’re sleeping with, or to assume that you’re interested in them sexually.”

That may go double for pansexuals. As Palmer puts it, there is often a knee-jerk assumption that they are all out swinging from chandeliers when “half the time you’re spending Saturday nights watching documentaries in your pyjamas”. When bi or pan people settle into long-term relationships, that can prompt hurtful assumptions that they have either finally “picked a side” or else may secretly hanker after whichever gender they are not currently with. “There’s this whole thing coming from the LGBT community: ‘Oh, you’re dating a girl, you must be gay,’” says Palmer. “But I’ve also had a partner’s parents saying: ‘Aren’t you scared Jezz is going to run off with a man?’ as if you’re always wanting what you can’t have, when it doesn’t really feel that way.”

Yet as Generation Z grow older, and become the dominant cultural influence, their belief that, as Meadowcroft-Lunn puts it, “people have the right to identify however they choose” is only likely to become more mainstream. Could we eventually reach a point where heterosexuality, or at least the uncompromising version at one end of the Kinsey scale, is no longer considered the norm and “coming out” as anything else is practically superfluous? “It’s still true that over 90% of the country identifies as straight, so I don’t want to overstate this,” says Harrad. “It’s more that awareness-raising is a virtuous circle - the more you know about minority sexualities and the more people you meet who identify as one of them, the less it feels like a big deal. And in an ideal world, why wouldn’t it be?”