Research by Masters and Johnson pointed towards a four stage sexual arousal cycle. The four stages are Excitement, Plateau, Orgasmic, and Resolution.
The Excitement stage can basically be summed up as desire, and can be a result of external stimuli ( Such as watching a couple erotically kissing. ) or internal stimuli. ( Fantasizing. ) In males this is the first stage, and required for males to have willing sex. It is one of only two known triggers for the Plateau phase for males.
The Plateau phase can roughly be summed up as the act of physical arousal prior to reaching an orgasm. This is the actual act of a man gaining an erection, and what many in the general public mistakenly believe to be the begging of sexual arousal. The Plateau phase can be triggered in one of two ways. One, either through the Excitement phase in which arousal is a physiological response to internal stimuli. The other is through the triggering of a physiological response to external stimuli. This second means of reaching the Plateau phase is what allows men to achieve an erection without actually desiring sex, as the act of arousal itself is a physiological response. Provided an external physical stimulation is applied to the penis during this stage, the man will move on to the third stage of the human sexual response cycle, the Orgasmic phase.
The Orgasmic phase is actually fairly complicated. The two earlier stages have various physic phenoma associated with them, such as an increased heart rate, higher body temperature, and faster breathing, but these are really ramped up during the the Orgasmic phase to their highest point during the entirety of the sexual response cycle, and a number of other responses, both physical and neurochemical. The additional physical response is that the penis and anus go through a series of contractions. Each contraction sends a 'jolt' through the nervous system causing the brain to release a plethora of chemicals in the brain ( Such as Oxycontin, Dopamine, and several others relating to both love and pleasure. In males, the primary chemical released during this phase is Dopamine which functions purely as a form of pleasure chemical. ) that signals you're experiencing pleasure in your genitals, loin region, and lower back. A few individuals also experience it in their upper thighs. The average man typically has 10 - 15 contractions during the course of an ejaculation, and the final contractions signal the beginning of the fourth, and final phase for men, the Resolution phase.
The Resolution phase triggers the end of the sexual response cycle, in which the body returns to normal by lowering it's heart rate and temperature ( Part of the lowering of temperature is the release of sweat, which is why you're almost always a sweaty mess afterwards. ) as well as slowing breathing. In men, this is the period in which the largest amount of Oxycontin released, which is the primary chemical responsible for being in love with someone. Unfortunately, our bodies are unable to distinguish between the person we love and a fling, which is why even short term relationships can end in heartache, and friends with benefits style relationships can and do often go wrong. ( Our brains operate on a very pattern like basis which attempts to associate particular emotions/sensations, and thus over time we associate people with whom we have intercourse with with the feeling of Dopamine and Oxycontin. This essentially [ Though admittedly simplistically. ] sums up how we fall into and maintain love with a person. We fall out of love when our brain stops associating those chemical rewards with that person. )
Their research, while extensive, did however have some notable flaws with it.
Probably the simplest and most readily discernible flaw is that they tried to compare increased vaginal lubrication that happens during female arousal as being the equivalent of a male erection. It was pointed out by Roy Levin that the appropriate comparison would be clitoral swelling, as the clitoris operates as the primary means of sexual stimulation for females.
Another large critique leveled against it is that Masters and Johnson argued that males are able to only achieve one orgasm through a given cycle. However, work by Hartman and Fithian as well as work by Zilbergeld demonstrated that while the vast majority of men are limited to one orgasm during an arousal, some men are able to skip the Resolution and Desire phases, and go straight from the Orgasmic phase directly to the Plateau phase.
Another problem exists that their cycle, while it works perfectly for males, doesn't seem to work well for females.
Their research demonstrated that a male can achieve up to 12 orgasms in an hour, which gives a decent length of time to go through all phases of the cycle. However, when applied to women, it doesn't seem to work as they recorded female participants in their research achieving up 127 orgasms within an hour, which means essentially they'd have to go through the entirety of the cycle from Excitement and Resolution in less than 30 seconds.
The Excitement phase was also shown to be demonstrably different between males and females. Men were, almost to the point of exclusivity, aroused by their preferred gender. A few men, such as bisexuals, demonstrated equal arousal by both genders. Women were found to be the opposite. The vast majority could become aroused by watching any combination of couples engaged in intimate acts ( Which has had some less reputable verification in women's consumption of porn which is dominated by three categories, the first two of which are applicable to this particular question, while those three categories are male on male, female on female, and domination. ) as well as sexual acts between non-human pairs. ( Yes a majority of women based upon the research are turned on by watching non-human pairs mating.
There are also other notable differences in regards to the the order in which the phases take place.
Under the four stage model used by Masters and Johnson there simply is not enough time to follow the phases in the order that was set out. There is an increasing body of evidence that the Excitement phase and the Plateau phase in women are in the reverse order. Yes, you read that correctly. Many women need to be stimulated before they want to be stimulated, whereas in men it's essentially the reverse.
By moving the Plateau phase to the first phase and moving the Excitement phase to the second phase we are able to overcome the problem of the frequency with which women can achieve orgasms, as well as their ability to have multiple orgasms in a row. Using a Plateau, Excitement, Orgasmic, Resolution model for women as it would implicate that continued stimulation of the clitoris would allow a woman to achieve orgasms without requiring a break in between orgasms, as she doesn't need to go through the Resolution phase to achieve the Plateau phase again. She can merely bypass it until she is ready to stop.
It further solves other problems such as the length of the Excitement phase and the time it takes to achieve the initial orgasm. A male's Excitement phase is generally short, at less than 30 minutes. A female's Excitement phase can, on average, range anywhere from a few minutes to several hours, with a few being recorded as having brain activity that signals desire lasting for in excess of twelve hours.
Further, the initial time it takes to achieve the very first orgasm from the beginning of the Plateau phase, when physical stimulation is applied, to the achievement of Orgasmic phase, for a male averages lasts on average from three to five minutes, whereas a female on average takes approximately twenty minutes. Biologically it seems to be a sort of trade off for the ability to achieve numerous orgasms in a significantly short period of time.