Sex is the most talked-about, joked about, thought-about issue in our culture. Every grown adult is expected to know how to do it, but beyond the basic mechanics we’re not taught about it and fiction is coy. We are not short of information on sexual practices – thank you, Fifty Shades of Grey – but there is a general absence of accurate detail of what happens to our bodies during, and as a result of, the act.

Yet sex is good for our mental and physical health. It lowers the heart rate and blood pressure. It may boost the immune system to protect us against infections and it certainly lowers stress. The NHS even recommends it, in a section tucked away on its website, where few are likely to find it, that advises: “Weekly sex might help fend off illness.”

The consultant obstetrician and gynaecologist Dr Leila Frodsham thinks we should be better educated about it. She’s even supporting a project to open a Vagina Museum in Camden, London – after all, there is a Penis Museum in Iceland. More information could make us healthier, happier and save the NHS lot of money, she believes.

“People who have difficulties with sex are much more likely to present with other problems,” says Frodsham. She would like to see more investment in sexual health as preventive medicine.

When hooking up is working out

Sex can be good exercise, although that rather depends on how energetically you go at it. A study in the open-access journal Plos One in 2013 found that healthy young heterosexual couples (wearing the equivalent of a Fitbit) burned about 85 calories during a moderately vigorous session, or 3.6 calories a minute. It’s unlikely to be enough. The NHS says: “Unless you’re having 150 minutes of orgasms a week, try cycling, brisk walking or dancing.”

Men who had more frequent orgasms had half the risk of dying over 10 years

Tales of men having heart attacks and expiring on the job are much exaggerated. Sex raises the heart rate, which is generally a good thing. A study in the British Medical Journal of 918 men in Wales in 1997 found that sex helped protect men’s health. Men who (admittedly from their own report) had more frequent orgasms had half the risk of dying over the 10 years of the study compared with those who had the least orgasms. As a general rule, if you are able to walk up two flights of stairs without chest pain, you are probably safe to have sex, experts say.

The key to many of the health benefits of sex is the love hormone – oxytocin. Also sometimes called the cuddle hormone, it can even be released when petting your dog. The same hormone causes contractions in childbirth and is in the pessaries given to induce labour. It’s even in sperm. It’s not a myth that sex can help an overdue baby get going. When she was working as an obstetrician, Frodsham says, male partners used to “leave grinning from ear to ear because I’d suggest having sex on all fours to make labour come on”. There’s plenty of oxytocin around when people have sex or even just get friendly. “Any touch releases oxytocin,” says Frodsham. Keeping up physical activity affects libido, she says. “If you don’t use it, you lose it.”

She doesn’t often see people with intrinsically low libido, she says. “But we do see people who kind of get into a sexual rut and it sort of disappears. I often encourage people to schedule sex. A lot of couples feel that it is not natural and it is forcing things, but sometimes you need to get them to become habitual so they can become spontaneous.”

Sex helps with sleep, and allows the brain to switch off. “If you are having sex, you should be getting into a zone where your brain is not in overdrive,” she says. It’s like mindfulness. “I don’t think there are many people who actually give themselves time to relax any more,” she says.

Prof Kaye Wellings, at the London School of Hygiene and Tropical Medicine, blames our busy lives for a decline in sexual activity in Britain. Her large recent study of 34,000 men and women, in the British Medical Journal, suggests we are having less sex than we were a decade or more ago. Half of the women and two-thirds of the men told researchers they would prefer to have sex more often. Wellings says the digital age is partly to blame. “We are bombarded with stimuli. I can see that the boundary between the public world and private life is getting weaker. You get home and continue working or continue shopping – everything except for good old-fashioned talking. You don’t feel close when you are on the phone.”

The sexual response, step by step

The best explanation of what actually happens during sex is still credited to two scientists who started work in 1957 – William Masters and Virginia Johnson – although later researchers have criticised parts of their work.

Masters and Johnson worked at Washington University in St Louis, Missouri. Masters convinced Johnson to have sex with him in the interests of research while he was married to someone else. He eventually divorced and they married in 1971, splitting up 20 years later. Together they founded the Masters and Johnson Institute where they carried out their research and trained therapists.

In a book called Human Sexual Response, published in 1966, they described a four-stage cycle in heterosexual sex. First is the excitement or arousal phase in response to kissing, petting or watching erotic movies. A small study by Roy Levin in 2006 found that almost 82% of women said that they were aroused by their nipples being fondled – and so did 52% of men.

Half to three-quarters of women get a sex flush, which can show as pink patches developing on the breasts and spreading around the body. About a quarter of men get it too, starting on the abdomen and spreading to the neck, face and back. Men quickly get an erection but may lose it and regain it during this phase.

Women’s sex organs swell. The clitoris, labia minora and the vagina all enlarge. The muscles around the opening of the vagina grow tighter, the uterus expands and lubricating fluid is produced. The breasts also swell and the nipples get hard.

Masters and Johnson say there is then a plateau phase, which in women is mostly more of the same. In men, muscles that control urine contract to prevent any mixing with semen and those at the base of the penis begin contracting. They may start to secrete some pre-seminal fluid.

The third stage is orgasm, in which the pelvic muscles contract and there is ejaculation. Women also have uterine and vaginal contractions. The sensation is the same whether brought about by clitoral stimulation or penetration.

Frodsham says about a third of women easily have orgasms from penetrative sex, a third sometimes do and a third never do. “I have never seen anything that could be a G-spot,” she says. But the clitoris is much larger than some people assume. “The clitoris actually surrounds the vagina. The protuberance is only 5% of the clitoris.”

Women can quickly orgasm again if stimulated, but men cannot. Last is the resolution phase, when everything returns to normal. Muscles relax and blood pressure drops. But, says Cynthia Graham, a professor in sexual and reproductive health at the University of Southampton, “we still don’t understand everything about what happens even though research has been going on since Masters and Johnson’s early lab studies”.

Take the female orgasm, for instance. “Women report so many different sensations. Some women describe orgasm in a much more focal way. Some describe it in a diffuse way with, for instance, a tingling down their legs. Some women describe losing consciousness.”

There is plenty of evidence of people wanting sex and having sex at older ages.

And then there is the male erection. A healthy man may have three to five erections in a night, each lasting around half an hour. The one many wake up with is the last of the series. The cause is unknown, but there are suggestions of a link with REM (rapid eye movement) sleep, when people are most likely to dream. Even in the daylight hours, erections are not necessarily under conscious control. Usually they are associated with sexual arousal, but not always.

There is an assumption that sexual desire and libido are strongest in the young and fade out as we age. But there is plenty of evidence of people wanting sex and having sex at older ages. For women, the menopause can be a real obstacle. The loss of oestrogen leads to vaginal and vulval dryness. Frodsham points out that hormonal treatments, from oestrogen tablets in pessaries delivered locally into the vagina to creams and gels, are safe and effective. But so is having regular sex, she says. It’s like exercising a muscle.

“There is very good evidence, particularly in menopausal women, that the more they have sex, the better their physiology is,” she says.

But she cautions against the current enthusiasm for promoting the health benefits of sex for all ages. “There can be a kind of pressure on older adults who don’t want to. A lot of older adults do, but not everybody. There’s no norm about sexual desire.”

However biologically similar we may have been at birth, the one thing that is certain is that sexual desire and preference – as well as means of achieving satisfaction – differ from one individual to the next. Frodsham, for one, thinks enhanced understanding could boost our mental and physical health. And, she believes, it needs to start early.

“Many schools present sex as something that is going to cause STIs and pregnancy,” she says. They’re missing something important, she adds: “They don’t talk about the very natural reason to want to have sex, which is pleasure.”