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."