It is 30 years since the release of When Harry Met Sally. Nora Ephron and Rob Reiner’s genre-defining romcom had so many hilarious, timeless lines, from: “How long do I have to lie here and hold her before I can get up and go home?” to: “When I get a new book, I read the last page first. That way, if I die before I finish I know how it comes out. That, my friend, is a dark side.” But one line that does seem to have aged is arguably the most famous, and the premise of the whole film: “Men and women can’t be friends because the sex part always gets in the way.” It is not just the heteronormativity that feels outdated; three decades on, speaking to some of the Harrys and Sallys of the millennial generation, the question now is less can they just be friends, and more, can they just have sex?

For Rachel, a bisexual woman in her early 30s, the answer is an enthusiastic yes, yes, yes! For about five years, she has gone through periods of regularly having sex with a friend she met at university, “with the agreement that we wouldn’t develop a deeper relationship,” she says. “We didn’t contact each other frequently in between dates or ask for the sort of emotional support you’d get from a partner. I cared about him, but I wasn’t dependent on his affection and I didn’t feel responsible for him beyond how you’d feel about a friend. And we’d have really good sex.”

Rachel always felt she knew exactly where they stood, because they talked about the nature of their relationship, discussing the limits of what they expected from each other. “When you are in an arrangement like this, you have to talk about things rather than make assumptions, and I really enjoyed how honest we were both able to be. I found it incredibly freeing that he didn’t ask anything from me.”

As someone who has never had this sort of relationship, I found it difficult at first to get my head around it – not because I felt judgmental, but because I felt admiring. I think you have to be quite emotionally mature to be able to accept something for what it is, without trying to turn it into something more, or denigrate it for not being something it is not.

“Relationships like this,” says Rachel, “where you are enjoying sex for what it is without making it represent something deeper, ask you to think about how sex usually functions in society.” She describes how, if you have sex with someone and get into a relationship with them, you are turning something that started off as a fun encounter into something that completely changes your life. You might end up spending most of your time with this person, making decisions about your life based on their input, using them as your main source of emotional support. “People assume that’s the natural trajectory, and sometimes that’s great – but sometimes it’s nice to just have sex with someone you like without those assumptions and expectations,” she says.

I ask her if there are any downsides: “Probably not.”

It may sound too good to be true, but for psychosexual therapist Kate Moyle, it does not have to be. “If both parties are really busy in their jobs, their social lives and family lives, and don’t have the available emotional space for a relationship, why isn’t this the perfect solution?” she asks. “You get to have sex with the same person, which can typically be quite satisfactory because you get to know each other and each other’s bodies, and there isn’t the emotional dependency and stress of dealing with someone’s feelings. You don’t lose your independence.”

She believes this kind of less demanding relationship is on the rise because of the lifestyles of young people. “We are a generation who seem to work such long hours, with the complete dissolving of nine-to-five because of technology.”

That is part of the appeal of sex-only relationships for Laura, in her late 20s, who began seeing her then-colleague Mark four years ago. “I have a busy life, a demanding job, and this situation works for me,” she says. “I don’t even know how I would go about getting into a relationship with someone right now, the time and energy you have to devote to that. It’s convenient to be able to say to someone at 11pm, ‘Are you around?’ You can’t really do that in a normal dating situation.”

Mark says: “It’s a bit like a relationship-lite. We usually see each other once a fortnight maximum, and the vibe is always quite intimate – even though it is understood that it will never be any more than what it is.” He adds: “At times, when I’ve felt unsure or anxious or worried or sad or lonely, it’s been incredibly comforting. And then at other times it’s just been really good fun – we do get on really well, and we have amazing sex.”

For Laura, “It’s always a bit more exciting, because you don’t fall into the same repetitive boring patterns of being in a relationship. You never get past that honeymoon period.” It also means she can avoid dating apps. “I don’t like modern dating – I don’t like sacrificing an evening to meet someone I’ll probably know instantly isn’t someone that I have any connection with, and then have a drink and be polite or whatever, for an allotted amount of time, before I can leave.”

But for Laura – unlike for Rachel – there is a downside. “There is something weirdly arrested about the whole situation. If you can never get past a certain point of closeness because you’ve imposed rules – verbally or non-verbally – on how close you can get, then there are going to be times where you feel that barrier.” You start wondering, she says, why don’t I know about all of your life? Why don’t you know my friends? It is not that this kind of relationship is better or worse than more traditional monogamous relationships, “but the nature of the thing is that it has its own limitations,” she says. “It’s also not something you can explain to friends and family. I’m seeing someone and it’s been going on a really long time but we’re not together – you can’t explain that to your mum, can you?” She laughs.

Things go wrong, in Moyle’s experience, when people change, or when they do not stick to the boundaries they have established at the start. “Difficulties tend to come up when one partner meets somebody new, or if they decide to end it. There is a sense of a relationship even if they want it not to be a relationship, because we have a form of a relationship with anyone we are regularly connecting with.”

This is what Mary found. She is a mother of three in her early 40s who divorced five years ago, and she has been having regular sex with a male friend. But it is now proving more complex than she had hoped. She has developed feelings of attachment for him, and he for her. This might sound like a Harry Met Sally happy ending, but, as she explains, it is not. “We weren’t supposed to. It’s complicated because he wants to spend more time with me, and I don’t want the same – I don’t want a relationship, as I am concentrating on my girls. It has been draining, as it’s getting in the way of our friendship. I think you have to lay down rules at the beginning and stick to them – or someone will get hurt.”

There is a name for two people having regular sex with each other on the understanding that it will not grow into a loving, committed relationship – in fact there are several names. “Friends with benefits” is one, “non-relationships” another. But, for the people I spoke to, none of these terms accurately encapsulates what is going on. For Emily Witt, the author of Future Sex, a book about contemporary sexuality, the name is important. “If you don’t have a name for what you’re doing, if you don’t have the words to describe your own reality, it increases your sense of alienation,” she says.

The best term she has found is “erotic friendship”, and, she says, erotic friendships have value. “In popular culture maybe they’re seen as cheap or disposable or a waste of time, but I think they’re places where you can learn a lot. You get to learn somebody’s sexual quirks and the diversity of what turns people on and what they want, you practise communicating your own desires and don’t just assume the person can intuit them. That experience really is worthwhile.”

Yet, Moyle says, these kinds of relationships have traditionally been stigmatised: people such as Rachel, Mary, Mark and Laura are depicted as people who don’t want to or can’t commit, people who want it all. “I guess it doesn’t fit with the historically expected monogamous model, therefore it’s considered ‘other’,” she says. “But we don’t have to conform to the traditional heteronormative model of man meets woman, they get engaged, married, have kids.”

This rings true for Rachel. “We still hold on to this idea of romantic love as a kind of happy ending for women,” she says. “If I’m sleeping with my friend whom I care about and who is kind to me, and I’m not in love with him, or making plans around our bond, I don’t think anybody’s being shortchanged – it just feels like a way to have fun together and enjoy closeness and human connection.” That idea of romantic love is what provides the happy ending of When Harry Met Sally, but, as Witt says, “that Hollywood thing, where any close friendship between people who might be sexually attracted to each other ends up in true love – that’s just not how it is”.

Perhaps if there were less stigma, and we knew more stories like Rachel’s, more single people would find themselves saying the film’s other most famous line: “I’ll have what she’s having.”

Names have been changed