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Sex is used both as a noun and a verb. Sexual reproduction requires a male semen to inseminate a female ovum or egg. Commonly sex, as a verb comprises a broad range of human behavior involving direct or indirect stimulation of the genitalia. Sex includes physiological, psychological, social and cultural aspects. The concept of what constitutes normal human sexual activity varies greatly between different human societies and cultures. A society's collective views on both gender and sexuality are reflected in its art and popular culture. In all societies there are laws or rules governing overt sexual conduct in public and some societies also regulate private sexual activities closely.
Physiological aspects 
The emotional turmoil of visual attraction is a strong and a sometimes confusing stimulus which occurs about the time of puberty. Sexual activity is normally considered pleasurable. For example, gently stimulating the genitalia is usually pleasurable both for men and women, and can lead to a climax or orgasm that is intensely pleasurable.
Children may experience sexual arousal, either by active stimulation of themselves or by engaging in activities that inadvertently arouse. For example, certain gymnastic activities may stimulate the sex organs. Where the stimulation is deliberate it is called masturbation, because it does not involve potentially reproductive activity.
Masturbation is an natural sexual practice that begins after puberty. It often begins alone and becomes a way of sexually pleasuring oneself without a partner. Masturbation can also be shared with a consenting partner, and this is usually called Mutual Masturbation.
Touching and kissing are important sexual behavior. Gentle touching can be sexually stimulating, even when this touch occurs on parts of the body not generally associated with sex like the hands, feet and neck. In modern Western culture, kissing is viewed as an expression of affection.
Generally kissing (osculation) may be either a mere social greeting (kissing cheeks, for example) or an intense sexual activity (usually kissing lips) French kissing involves stimulating lips and tongues with both partners' mouths open.
Sexuality is an expression of gender. The reason for a person's sexual orientation (preference for heterosexual or homosexual activity) is not known. Many theories have been proposed including: genetic factors; hormone exposure during pregnancy and personal experimentation in early life. However no reliable research has provided confirmation of any of these theories. Some people prefer different activities with same sex partners and members of the opposite sex, and are described as bi-sexual.
When human sexual function is impaired it is called sexual dysfunction. One common sexual dysfunction is a decrease in sex drive or libido. This can happen to either men or women and is sometimes brought about by stress. Men can suffer from erectile dysfunction. Erectile dysfunction is sometimes related to cardiovascular health, and it can often be treated effectively with medicines.
- The sebaceous glands in the mouth and lips release sexually stimulating chemicals. Smoking, alcohol, breath fresheners, and garlic mask these chemicals, and therefore may make it less enjoyable to kiss.
In the brain, the area for orgasms is next to the area for feet sensation. Therefore many people experience pleasure from foot washing or massage.
Sexually Transmitted Diseases (STDs) 
There are a number of harmful diseases which are transmitted through body fluids (including semen, vaginal fluid, and pre-ejaculatory fluid). These are referred to as STDs, short for "sexually transmitted diseases". Monogamy (having a single sex partner), where both partners are monogamous, will limit exposure to STDs. Individuals who have had two, three, or four partners have a 3-5% likelihood of contracting a STD. This percentage jumps ten times (to 28-35%) for promiscuous men and women with more than twenty sexual partners. Safe sex is a set of techniques and behaviors designed to reduce the likelihood of the transmission of STDs. These behaviors include limiting sex partners and the use of barrier protection like condoms.
Sexually transmitted diseases (STDs) are virulent and so should be treated early as soon as you are aware of a problem. Untreated STDs have a wide range of debilitating and enduring health effects. Regular check-ups may detect diseases before any obvious symptoms appear, and all diseases are easier to treat when detected early.
Many people with STDs are symptom-free, unaware of their infections, and untreated. Consequently a majority of people who are infected are unaware of it.  Some people who know they have an STD will not admit it. According to one study, 75% of HIV-positive men and women who know they're infected don't inform casual sex partners. Because of this, it is important for partners to communicate with each other about their history, and about the very real risk of STDs.
Birth control 
Children are a natural result of a male penis ejaculating semen inside or near a female vagina, and is properly called sexual intercourse. In some cultures sexual activity is regarded as a harmless amusement between consenting adults and children are not wanted. Casual sexual activity can however be emotionally damaging to some people. Even within a stable loving relationship the conception of children may have to be regulated and with proper care and planning modern contraceptives are generally effective. No method or combination of methods except sterilisation is 100% reliable.
Most hormonal contraceptive options that exist are for use by women. The choices are many and varied, including:
- Condoms, which unlike the others can be used by men or women. Male condoms are generally more effective in practice than female condoms in preventing conception. When used correctly male condoms are effective in preventing the spread of STDs. Male condoms that are correctly used break about 2% of the time. Male condoms are about 98% effective in preventing pregnancy and female condoms about 75%.
- Birth control pills are used by women and they work by elevating hormone levels so that ovulation is prevented. When these pills are used exactly according to instructions they are 99.7% effective in preventing pregnancy. However with imperfect use they are 92% effective. They do not offer any protection against STDs.
- "Emergency contraceptives" or "morning-after pills" are hormone pills which can be used within the first 72 hours after sex to prevent pregnancy with a 89% effectiveness..
- The Rhythm Method, or limiting sexual activity to a time when the woman is not ovulating was once promoted as a natural method of birth control acceptable to people of strong religious objections to modern contraceptives. It is thought to be on average only about 75% effective in practice , and does not offer protection against STDs.
Social and cultural aspects 
Sexuality is a fundamental human behavior that dramatically affects relationships between human beings. Sexual behaviors can be governed by implied rules of behavior and the status quo. Who should be chosen as a sexual partner, and when sexual behaviors should occur are often governed by social conventions. Sexuality influences social norms and society in turn influences the manner in which sexuality can be expressed. Various cultures and societies have laws that govern sex and sexuality.
One way that society influences sexuality is through formal social institutions called marriage. Monogomous marriage is most common, involving one man and one woman. Poligomous marrages are permitted in some cultures and usually involve one man and several women. A societal benefit of marriage is that it formalizes the bond between sexual partners and can form a stable pair of individuals who are committed to raising children. Some studies have shown benefits of a stable marriage for the children of the married pair.
Marriage can also have benefits for the individuals who marry. According to one study, a large percentage of married people in the US say they're very or extremely satisfied with their sex lives.. Twice as many single individuals suffer from stress, compared to married individuals (25% vs. 13%). Married men and married women have the same stress levels, on average.
One social and legal convention is consent. The nature of consent varies across cultures, but the essence is that both partners must be willing to enter into the activity and in the right state of mind to make the decision. In some legal systems the absence of consent may be considered a crime - that is, actual not presumed consent is required.
The age at which a person can legally give consent varies across the world. Most countries have a minimum age of consent between 13 and 18. Some have a minimum age of 12 years old, some as high as 20 years old and other countries require you to be married before engaging in sexual intercourse. In addition a few countries also have a different age limit for men and women. Many countries ban homosexual intercourse outright and other countries have a higher age of consent for homosexual intercourse (although all European Union countries have now equalized the minimum age for heterosexual and homosexual intercourse).
According to the law in many US states, people who have sexual intercourse with a drunk partner are committing rape .
- Lloyd-Elliot, Martin. Secrets of Sexual Body Language. (Ulysses, 1995, ISBN 1569750602), p. 86.
- Amen, Daniel. Making a Good Brain Great: The Amen Clinic Program for Achieving and Sustaining Optimal Mental Performance. (Harmony, 2005, ISBN 1400082080).
- Michael, Robert T., Gagnon, John H., Laumann, Edward O., Kolata, Gina. Sex In America: A Definitive Survey (Little Brown, 1994, ISBN 0-316-07524-8), p. 193.
- Chase, Marilyn. "Sexually Transmitted Diseases Appear Sharply Underreported," The Wall Street Journal, February 13, 2002, p. B9.
- Sternberg, Steve. "Most HIV-positive males in the USA don't know it." USA TODAY, July 9, 2002, http://usatoday.com/news/healthscience/health/aids/2002-07-08-us-aids.htm
- Norton, Amy. "HIV-Positive May Delay Telling Casual Sex Partners", Reuters Health, October 30, 2001; research by Megan E. O'Brien of Tulane University, presented to the American Public Health Association.
- | "FDA OKs "Morning After" pill without a prescription". Retrieved 3-17-07.
- Gregory Acs, Sandi Nelson What do "I Do"s Do? Urban Institute
- Laumann, Edward O., Gagnon, John H., Michael, Robert T., Michaels, Stuart. The Social Organization Of Sexuality: Sexual Practices In The United States (University of Chicago, 1994, ISBN 0-226-46957-3), p. 364.
- de Vaus, David. Family Matters, winter 2002.
|About This Book · Q&A · Recommended Books|
|The Science:||The Evolution of the Human Brain · How Women Select Men · How Men Select Women · How Our Ancestors Lived · Monogamy and Polygamy · Hormones · Communication Styles|
|Life Stages:||Childhood—Seeking Unconditional Love · Adolescence—Seeking Romantic Love · Adulthood—Families And Forgiveness · Agape—Altruistic Love|
|Practical Advice:||Where Couples Met · Flirting · How to Write a Personal Ad · Dating · Sex · Becoming a Couple · Conflict In Relationships|
|Personality Types:||Emotional Control Systems · Zeus-Hera · Poseidon-Athena · Apollo-Artemis · Hermes-Hestia · Ares-Hephaestus-Aphrodite · Dionysus-Demeter · Hades-Persephone|