Masters and Johnson were primarily interested in studying the biology of sexuality (i.e., what happens when you have sex) and so in the late 1960s they set up a lab and actually observed individuals engaging in sexual behavior. Their work resulted in the establishment of the Sexual Response Cycle which continues to shape the way in which we view sexual functioning, even today. The sequence of phases in Masters and Johnson's model of sexual response is:
Excitement phase: Physiological arousal increases quickly. Muscle tension, heart rate, blood pressure, and breathing rate increase. In men, the penis gets erect and the testes swell. In women, the clitoris hardens and swells, the vaginal lips open, and the vagina lubricates.
Plateau phase: Physiological arousal continues. In women, the clitoris retracts under the clitoral hood. Men may secrete a small amount of fluid from the penis.
Orgasm phase: Physiological arousal peaks. Men ejaculate seminal fluid. Both men and women experience muscular contractions in the pelvic area, along with a sensation of pleasure.
Resolution phase: Physiological responses return to normal levels. Men then go through a refractory period that can vary in length, during which they are not responsive to stimulation. The refractory period tends to get longer as men age.
Some sex scholars have suggested that an important flaw in the Masters and Johnson research on human sexual response is their inclusion of only orgasmic subjects in their sample. Masters and Johnson studied a biased sample of people. The sample included only people who were both willing and able to perform sexual acts in a laboratory setting.
Masters and Johnson didn’t pay attention to individual differences. In reality, people’s sexual responses vary according to factors such as age, amount of sexual experience, and cultural background.
In 1979, Dr. Helen Singer Kaplan developed her own theory of the the Sexual Response cycle. She propoesed that the first stage in the cycle is "Desire," as opposed to excitement. She believed that desire involved the mind. She expressed her thoughts in that while there are reflexive tendancies of the body brought on by arousal, the mind and the importance of perception, memory, and judgement play huge roles in the way the body responds to sexual intamacy.
The second stage, acoording to Kaplan is continuous with the the plateau stage, in that sexual excitement continues to build. Kaplan believes that you can't separate the excitement and plateau phases because they are very closely related and occur in the same fashion.
The last and most important stage is the orgasm itself. This is where everything came together, both emotionally and physically for both the man and the woman. They both experience sensations of extreme pleasure followed by strong sense of togetherness and closeness. Kaplan decided that the resolution phase was not needed in the cycle as the orgasm marked the pinnacle of the cycle.