Shannon Scrivens, 18, (pictured) can remember every detail of her first sexual encounter with another woman

Shannon Scrivens can remember every detail of her first sexual encounter with another woman: the vodka she had beforehand to release her inhibitions; the smell of jasmine perfume; the David Bowie album that played in the background.

She describes the moment afterwards when, hurriedly putting her clothes back on before her mother returned from work, she and her female friend agreed what had happened would not ruin their otherwise platonic relationship.

But what 18-year-old Shannon isn’t as clear about is whether she actually enjoyed the experience. ‘It was disappointing because I think my feelings towards women are romantic, rather than sexual,’ she says. ‘But I am still attracted to both sexes and would describe myself as bi.’

As confusing as this might sound to some — and at times when discussing her sexuality Shannon herself sounds confused — she is far from alone. According to recent figures from the Office for National Statistics, the number of British people defining themselves as bisexual has risen 45 per cent in just three years.

This alleged increase is most notable in the young — for the first time, more 16 to 24-year-olds describe themselves as bisexual than those who say they are gay or lesbian combined. A survey by YouGov last year found that half of young people declared themselves as something other than 100 per cent heterosexual.

So what has sparked this staggering shift in the sexual tastes of our younger generation? Can half of all young people really be sexually fluid? Or are they simply labelling themselves bisexual in order to fit in with what is undoubtedly a fashionable trend?

One glance at the growing list of young celebrities who claim to be attracted to both sexes would certainly help explain this seismic shift in sexual attitudes.

There’s pop stars Miley Cyrus, 23, and Lady Gaga, 30, to name but two hugely influential females who readily discuss their ambiguous sexual orientation in public.

Then there are a growing number of TV shows, such as Game Of Thrones, which feature bisexual characters. So what does all this really signify?

‘It is not that sexual desire has hugely shifted in recent decades,’ says Dr Mark McCormack, co-director at the Centre for Sex, Gender and Sexualities at Durham University.

‘Older generations had same-sex desires growing up, but because there wasn’t as much cultural awareness they assumed they were gay.

‘However, the idea that sexuality isn’t categorical but a continuum is increasingly recognised. Children are coming out earlier, at school, sometimes as young as 12 or 13.’

And what do the middle-aged parents of bisexuals, many of whom were brought up to believe sex should remain between a man and a woman, make of it all?

Shannon’s mother, Tina, 45, tries her best to understand but admits to struggling with her daughter’s sexual orientation. ‘When I was a teenager, homosexuality was kept quiet and bisexuality wasn’t even mentioned as a concept,’ says Tina, a carer.

‘I have told Shannon I think it’s wrong. I just don’t think it’s natural.’

Nonetheless, Shannon, an articulate and sociable university student, is undeterred. ‘I think Mum is being stupid and I have told her to get over it,’ she retorts.

Lewis Oakley, 25, (pictured) had girlfriends in his teens and assumed he was heterosexual until he was 19 years old and in his first term studying advertising at Middlesex University

In fact, Shannon is convinced that it is the people like her mother who are the ones fooling themselves.

‘Very few people are completely straight or gay,’ she insists.

Shannon says she has always been attracted to girls on a subliminal level.

‘As a child I thought they were pretty and when a friend told me she was bisexual when I was 11, it made me realise I was, too.’

But in her quiet home town of Sandy, in Bedfordshire, sexuality is rarely discussed.

‘Our classmates thought my friend’s bisexuality was weird and I was shy and worried what people would think, so I pushed the thought away. I told myself I just admired girls and only liked boys,’ she says.

It wasn’t until April last year, when Shannon was 17 and had yet to kiss a girl, that her mother found out she had feelings for both.

Tina, whose 18-year marriage to Shannon’s father, Robert, ended two years ago, says her suspicions were raised after she read cryptic Facebook posts Shannon had written about girls she admired.

‘She came into my room to talk about sex and I admitted it,’ says Shannon. ‘Mum was confused and thought it was a phase or a cover-up for the fact I was a lesbian — neither of which is true.

‘I’m not bothered that she said it was wrong. Perhaps she was hurt that I hadn’t confided in her before and I suspect because she has no experience of bisexuality she found it unsettling, but I said she would have to learn to accept it.’

Shannon told friends shortly afterwards. ‘I sat them down and said: “I have something to tell you. I fancy girls, too.” They hugged me and said it was OK.’

Shannon believes that because she connects more with women on an emotional level — something many straight women experience — she is what she calls ‘bi-romantic’.

Despite coming out, she didn’t start dating girls — ‘in my area there aren’t many bisexuals so it was hard to find someone’ — and instead lost her virginity in 2015 to a male schoolmate she dated for six months before he ended the relationship this year.

Her sexual experience with a woman happened spontaneously this May, when she and a lesbian friend ended up in bed together after hanging out after school.

‘I found it strange, but maybe the reason it felt weird was because she was a friend,’ says Shannon.

Perhaps understandably, she didn’t tell her mother, though Tina insists she would allow Shannon to have a lesbian relationship under her roof ‘if she was serious, but not if she was only experimenting’.

Despite the trend for girls to upload pictures to social media of themselves kissing other girls, Shannon is adamant that her flirtations with females are genuine and not as a result of peer pressure or for the titillation of men.

‘I’ve never met anyone who is pretending — that would be stupid,’ she says.

Francesca Dean, 19, (pictured) says she realised she was bisexual aged 15

Last month, Shannon started studying English at the University of Salford in Manchester, where the liberal sexual landscape is in stark contrast to her home town and, she says, ‘literally everyone is bisexual’.

She believes most people sit somewhere on a scale of sliding sexuality, and that something as innocuous as admiring a woman’s figure indicates bisexual feelings.

‘Even admiring another girl’s bottom isn’t exactly the straightest thing to do, is it? I move up and down the spectrum and am open to the idea of sleeping with another woman, ideally one in her 30s.

‘Older women are more caring. I will be attracted to women for life, but I will probably end up with a man as it is more socially acceptable. Plus I definitely want children, which is more straightforward with a man.’

So what is life like for young male bisexuals in modern Britain? Lewis Oakley, 25, had girlfriends in his teens and assumed he was heterosexual until he was 19 years old and in his first term studying advertising at Middlesex University.

‘Life in London was a lot more open compared with my upbringing in the West Midlands,’ he says.

‘There were lots of gay men in my social circle and after a party one night, a guy I met kissed me.

‘I realised I might be open to exploration, but didn’t immediately see myself as bisexual and didn’t want to tell anyone until I was certain.’

It took six months of discreet flirting with men as well as women in bars before he was convinced — and then he told his mum, Andrea, 45, a teaching assistant.

‘She was wary at first — she hadn’t much exposure to bisexuality,’ he says. ‘She thought it was a phase rather than a fully fledged orientation, but soon realised I was serious and she respected my feelings.’

His heterosexual friends were similarly supportive. The only people who had an issue with it were in the gay community.

‘They are quite bi-phobic and don’t see bisexuality as a legitimate orientation, but as a way of coming out as a homosexual,’ he says.

Lewis’s first relationship with a man was when he was 21, with a model agent he met on Twitter.

It lasted two years until, he says, they ‘grew apart’.

He started dating his girlfriend, Laura, also a teaching assistant, seven months later.

Already a friend and aware of his bisexuality, she says that far from a deterrent, Lewis’s sexual orientation improves their relationship.

‘As he’s dated men and women he doesn’t expect or demand stereotypical gender-based roles from me just because I’m a woman,’ says Laura, 27.

Although Lewis concedes he has more options in terms of sexual partners, he rejects the suggestion that bisexuals are incapable of monogamy, as does Laura: ‘People tell me Lewis will get tired of women and want to explore men.

‘But sexual orientation has nothing to do with fidelity, and just because you have the capacity to be attracted to both sexes it doesn’t mean you have to constantly switch between the two.’

Francesca Dean, 19, says she realised she was bisexual aged 15. ‘After a party, I hugged a female friend goodbye and realised I felt a level of attraction I’d only felt with boys before,’ she says.

Something previous generations might have regarded simply as a harmless crush became a huge, troubling issue for her.

‘I cried all the way home. I was worried I wouldn’t be accepted, that my female friends wouldn’t want to sit next to me in class.

‘I felt confused and scared and tried to overcompensate by flirting with boys more than I’d otherwise have done. I told only two of my closest friends and worried about telling my parents. I didn’t want to make them feel uncomfortable.’

In the end she blurted it out to her mother, Tracy, 54, six months later in April 2012.

‘She said it was starting to become trendy for young people to say they were bisexual and laughed,’ says Francesca. ‘I was thrown. There was a part of her that thought it wasn’t true. She thought it was a phase.’

In my generation, being gay wasn’t talked about widely and bisexuality wasn’t something I had any experience of at all

Tracy, an IT consultant from Brighton, says: ‘It wasn’t a laugh of ridicule. It just wasn’t what I had expected to hear.

‘In my generation, being gay wasn’t talked about widely and bisexuality wasn’t something I had any experience of at all. I realised I had to take a step back and let her get on with it.

‘Now I’m proud of her,’ adds Tracy, who is married to Olivo, 54, a university researcher.

Shortly afterwards, privately educated Francesca announced her bisexuality on Facebook.

‘I wrote that I didn’t want to hide and needed to come out as bisexual,’ she says. ‘Most of my friends were supportive, although some thought that I was trying to get attention.

‘One guy started calling me Miss Bisexual, which was disrespectful, but the girls I shared a dormitory with didn’t treat me any differently.’

Aged 17, Francesca kissed a girl for the first time during a vodka-fuelled game of spin the bottle at a party.

‘She was straight and it felt much the same as kissing a boy,’ she says. ‘It made me feel more comfortable in my sexuality.’

Other than one year-long relationship with a boy when she was 13, Francesca, now a drama student at university in London, has spent her adolescence single.

‘I am attracted to boys and girls more at different times,’ she says.

‘Statistically, I am more likely to end up with a man, but I would be happy to settle with a woman.’

She says several of her friends have also come out as bisexual.

‘Our generation is rejecting the boxes previous generations have laid out for us,’ she says.