It would perhaps be stating the obvious to state that it is sensual pleasure primarily that propels a human being into having sex. It would be worthwhile here, to note the changes that take place in both men and women before, during and after sexual intercourse. These include changes in the genitals as well as other parts of the body. In both sexes, the cycle of sexual response could be divided into four phases to ensure safe and effective intercourse, and one that could lead to the fertilization of the egg (ovum). They are as follows:

Excitement Phase
Duration
It could vary from just a few minutes to hours on end, depending on the techniques adopted, and other external factors such as lack of distraction.
In males
When in a state of arousal, blood is pumped into three cylinder–shaped bundles of spongy tissues in the penis. This makes the penis stiff and erect, thereby making it stand erect at an angle. The scrotal sac is then elevated slightly owing to its muscular contractions, thereby bringing the testes upwards.
In females
The vaginal walls stretch and secrete a lubricating fluid. The uterus (womb) rises in the pelvis. The labia majora (outer vaginal lips) open up slightly. The labia minora (inner vaginal lips) thicken. The clitoris grows in size, swells and stiffens.

Plateau Phase
Duration of the phase
The plateau phase is usually as brief as it is intense.
In males
The penis grows slightly larger. The head of the penis (glans penis) becomes reddish and shiny. A few drops of a sticky fluid (mucus) emerges from the opening at the tip (meatus). The testes are drawn up even further.
In females
The outer portion of the vagina swells with more blood flowing into it. The swollen outer vagina and labia minora (inner lips) form a long elevation known as an orgasmic platform. The clitoris hides behind its hood.

Orgasmic Phase
Duration of the phase
The orgasmic phase is the shortest. It lasts for merely a few seconds.
In males
A series of involuntary rhythmic contractions in the testes and the penis force out semen from the penis in quick spurts. This renders the male great sexual pleasure. This process of expulsion of semen from the penis is known as ejaculation.
In females
The orgasmic platform contracts rhythmically anything between three to 15 times at short intervals of about 0.8 seconds. These rhythmic contractions of the orgasmic platform along with structures in the pelvis provide the female intense sexual pleasure.

Resolution Phase
Duration of the phase
Just about 10 seconds, that’s all.
In males
This phase goes through two stages. In the first stage, the penis shrinks to half its fully erect size. In the second, it returns to its normal flaccid size from the semi–erect stage. The second stage takes a little longer than the first. As the penis shrinks, the scrotum becomes loose and the testes descend.
In females
About 10 seconds after vaginal contractions, the clitoris returns to its normal position. The whole vagina returns to its normal color, size and position in about 15 minutes. The uterus slowly regains its normal position which happens in about 20 minutes. The inner and outer lips of the vagina revert to their original color and size too.

Changes in other parts of the body
Reddish spots called sex flush appear on the abdomen, throat and breasts. These go away about five minutes after orgasm. Muscular tension takes place, causing the nipples to become erect, the thighs become tense, the back becomes arched, the nostrils get flared etc. All this fades away within five minutes after orgasm. A thin film of perspiration often covers the whole body after orgasm. The heart rate, breathing and blood pressure, all rise in both sexes during the excitement, plateau and orgasmic phases. These return to normal soon after orgasm.