PE Definition by Alfred Kinsey
It's been widely reported that Alfred Kinsey discovered that three quarters of men ejaculated within 2 minutes of penetration in more than 50% of their sexual encounters with their partner.
It's interesting that in light of this clear statement, Masters and Johnson came up with a definition of PE as an ejaculation that occurred before the woman had reached orgasm. You can see, superficially at least, that such a conclusion could be inferred from Alfred Kinsey's findings about male sexual performance, but Masters and Johnson were certainly guilty of making the assumption that orgasm through intercourse would otherwise be a natural outcome of coitus for the majority, if not all, women.
The reality is that modern research demonstrates not much has changed since Kinsey's days, despite a plethora of works on the Internet claiming that premature ejaculation affects only 20 to 30% of men. I believe the prevalence of PE is much higher than this. For example, research by the University of Kohn in Germany demonstrated that the majority of men who describe themselves premature ejaculation lasted for an average time of 2 minutes and 30 seconds from the beginning of intercourse to the point ejaculation, whilst those men who did not describe themselves as having premature ejaculation managed to last an average time of only 3 minutes.
There is absolutely no reason to doubt the accuracy of this research, and what it says to me is that premature ejaculation is actually the normal condition of the human male. Of course, that's a statement which only becomes meaningful when a definition of premature ejaculation is supplied! From a functional point of view such as Masters and Johnson were adopting, it's very tempting to relate male sexual performance to the achievement of female orgasm. If we resist that temptation, and instead look at levels of partner satisfaction, a good definition becomes easier to formulate.
We could move from Alfred Kinsey's definition of premature ejaculation - that the man's ejaculation happens within two minutes of the start of intercourse - to a more meaningful definition, something like this: that the man ejaculates before either he or his partner are fully sexually satisfied, leaving them wishing that intercourse could continue for longer.
You see that this definition doesn't relate the occurrence of premature ejaculation to the female orgasm, it simply relates it to partner satisfaction with the sexual interaction. Many women report that they find intercourse enjoyable mostly because of the deep emotional connection that they feel with their male partner, whether or not they achieve orgasm. There is, of course, an element of physical pleasure as well from the stimulation of the vagina during sexual thrusting, and it's not unreasonable to suggest that the longer this lasts, the more satisfaction a woman will experience during intercourse, regardless again of whether she orgasms or not. For the man, satisfaction is more likely to be around the issue of how he feels about his sexual performance; if he ejaculates quickly he's likely to report feeling less masculine, less satisfied, and less confident around his sexuality and his ability to please women.
In conclusion, you can see how the components of a practical, working definition of premature ejaculation begin to evolve when you combine both works of Alfred Kinsey and Masters and Johnson.