PE Definition by Masters and Johnson
Masters and Johnson, or more exactly Dr William H Masters and Virginia E Johnson, have often been described as pioneering sex researchers. What was different about their approach was that they were uninhibited in investigating human sexuality, and in regarding it as a normal aspect of our existence. This fundamental shift in approach from previous decades allowed them to explore human sexual dysfunction, to diagnosis sexual dysfunction, and to treat it, in a much more effective way than had previously been the case. Another difference in their approach was to take people into a residential setting, and give them training and education on how to overcome sexual dysfunctions and problems over a short period of time in an intense sequence of consultations.
They began their research at Washington University, and continued it at the independent foundation known as the Reproductive Biology Research Foundation in 1964, later known as the Masters and Johnson Institute from 1978 onwards. Their work is now documented by the Kinsey Institute. (See Kinsey's definition of premature ejaculation here.)
A brief history of their research work is instructive in understanding how they could investigate premature ejaculation and come to what was effectively the first formal definition of premature ejaculation that had been attempted. They began by recording laboratory data on both the anatomy and physiology of men and women during sexual stimulation. Using the sexually uninhibited approach referred to above, they watched over 10,000 cycles of human sexual response, and as a result were able to define previously unknown aspects of sexual arousal such as the exact way in which woman's vagina lubricated prior to intercourse and during sexual arousal. They also investigated the nature of human orgasm, demonstrating similarity between clitorally and vaginally produced orgasms, and demonstrating the capacity of women to have multiple orgasms. This work dispelled a great number of long-standing preconceptions about human sexuality.
Their best-known books were Human Sexual Response and Human Sexual Inadequacy, published in 1966 and 1970. (The partnership began actually in 1957, when Masters hired Virginia Johnson as a research assistant.) Although Kinsey predated them in publishing work on human sexuality (specifically in his Kinsey reports of Sexual Behavior In The Human Male in 1948, and Sexual Behavior In The Human Female in 1953), Kinsey's work had mostly been reportage of the frequency with which certain sexual behaviors were found in the human population.
Masters and Johnson were actually interested in studying the function and form of human sexual response, using such previously unknown techniques as observation and measurement of masturbation and sexual intercourse as it took place in the laboratory. This produced physiological data, but it also enabled them to present both sexual behavior and sexual dysfunction, including premature ejaculation, in a way that emphasized the fact that it was both a healthy and a natural part of human existence, and formed an essential part of the pleasure and intimacy of sexual relationships. In this framework, it seems completely natural they would have undertaken the investigation of sexual dysfunctions like premature ejaculation, since this can be extremely disruptive to both the sexual aspects of a relationship, and to the relationship in a wider setting, outside the bedroom, as were.
It's also clear that the observational work which they carried out would have been extremely useful in helping to define premature ejaculation, although in retrospect we can see that their definition was certainly framed within the limitations of thinking at the time. More exactly, the belief that most women could reach orgasm during sexual intercourse as a result of the man's thrusting alone appears to have framed their thinking around the definition of premature ejaculation.
They defined premature ejaculation as a man's inability to delay ejaculation for long enough so that the woman could achieve orgasm during intercourse 50% of the time.
The presumption that a woman could reach orgasm during intercourse as a matter of routine underlies this definition, and seriously weakens it. What we can say with certainty is that a number of women could reach orgasm if their man could delay ejaculation and continue thrusting vigorously for long enough, although it's necessary for a woman to be highly aroused before intercourse starts, and for her to be very familiar with the process of reaching orgasm through vaginal stimulation, for this to happen.