In mammals the strict biological definition of sexual
intercourse, or coitus, is the insertion of the male's erect penis
into the female's vagina, to impregnate the female. The traditional definition of human sexual
intercourse is also penile-vaginal penetration for the purpose of conceiving a child. Some people, however, use the term more broadly to include a variety of other human sexual behaviors that may involve other parts of the body, may or may not include penetration, and may be performed by members of the same gender.
Humans engage in sexual intercourse for a variety of reasons beyond the need to conceive a child. Sexual intercourse can be an intimate form of communication between people, giving
physical and emotional pleasure, ecstasy, and orgasm. Orgasm is a complex reaction involving physical reflexes and powerful emotions. It often lasts from a few seconds to a minute or more and is followed by a feeling of physical and emotional relaxation and relief. Female orgasm exists in a great variety of forms, ranging from a single episode of mildly pleasurable rhythmic contraction of the uterine and vaginal walls to multiple episodes of extreme intensity that involve the entire body and can last for minutes or reoccur for hours. Male orgasm also exists in a great variety of intensity, duration and frequency patterns. For men, orgasm is usually precipitated by a series of penile thrusts and accompanied by rhythmic contractions of the prostate and muscles surrounding the penis, elevation of the testes, and ejaculation.
SEXUAL DESIRE
The desire for sexual expression varies greatly from one individual to another. A person's desire for sexual expression is influenced by a multitude of factors, ranging from feelings of attractiveness, the availability of a partner, level of personal power, economics, cultural attitudes, genetics, religious beliefs, age, level of health, and gender. Research reports have documented that humans have the capacity to enjoy and engage in sexual activity throughout their lives. Almost all human beings express themselves sexually (ranging from several times a day to only a few times in a lifetime) through masturbation, intercourse, and a variety of other activities, depending on their level of desire and the other factors that influence their behavior. The desire component of sexuality has been incorporated into alternate models of the sexual response cycle by several researchers. One model describes four phases, beginning with desire, combines the excitement and plateau phases, and includes orgasm and resolution, while another five-phase model includes desire, arousal, physiological readiness, orgasm, and satisfaction. A third model combines the physiological and psychological dimensions of sexual response by overlapping the four-phase model with the psychological components of seduction, sensations, surrender, and reflection.