Coitus (Latin: coitus, „copulation"), also called intercourse, sex or copulation, is principally the insertion of the man's erect penis into the woman's aroused vagina. [1] Other forms of sexual intercourse include anal sex, oral sex and fingering between hetero and homosexual sex partners. [2] [SEXUALITY]

Since the 1950's there are extensive studies on sexual intercourse by Kinsey [HETEROSEXUALITY] and Masters & Johnson [CLITORIS]. One of their most enduring and important work aspects has been the four stage model of sexual response, which they described as the human sexual response cycle and defined as: [2] [3] [4] [5]

1. Excitement phase (initial arousal): increased circulation of genitals (lubrication of vagina and erection of penis), increased blood pressure and heart rate

2. Plateau phase: a continuation of the same changes evident in the excitement stage, the phase is characterized by an increased circulation in genital region (the outer third of the vagina tightens reducing the diameter of the opening of the vagina and the uterus swells)

3. Orgasmic phase: conclusion of the plateau phase of the sexual response cycle, accompanied by quick cycles of muscle contraction (anal, vaginal), breathing and heart rate are increased even further, possible ejaculation in men and women [FEMALE EJACULATION]; generally euphoric sensation [ORGASM]

4. Resolution phase: after orgasm, allows the muscles to relax, blood pressure to drop and the body to slow down from its excited state

These phases as well as the sensation of an orgasm can vary from individual to individual. [3] [4] [5] [ORGASM]

Culture
Vaginal intercourse between man and woman is often regarded as the most natural form of sexuality because it serves reproduction. Thus, in many cultures and religions the heterosexual act is something sacred. Other forms, such as anal intercourse, are often considered harmful and are rather tolerated than desirable. [2]

A different point of view is shared by several famous experts, physicians and sexologists, such as Christopher Ryan, the author of "Sex at Dawn" (2010), sexologist and author Dr. Elia Bragagna, Dr. Beier, a leading expert for sexual medicine in German-speaking Europe, MA. Wolfgang Kostenwein, Institue for sex education in Vienna et al. In their opinion, humans are highly sexual beings who have sex in order to establish relations and intimacy. For many, sexuality is an expression of pleasure, affection and love instead of a means to reproduce. Due to the depreciation of different forms of sexuality, a pleasurable and positive attitude towards relationships is prevented. The consequences are a suppressed sexuality which might find expression in depression, frustration or even sexual violence. [2] The fixation on reproduction ignores the essential qualities of sexuality like relationship and pleasure gain and locks away the chance of a highly human pleasure. [5]