Extramarital sex occurs when a married person engages in sexual activity with someone other than his or her spouse. From a different perspective, it also applies to a single person having sex with a married person.
Where extramarital sexual relations breach a sexual norm, it may also be referred to as adultery (sexual acts between a married person and a person other than the spouse), fornication (sexual acts between unmarried people), philandery, or infidelity. These terms may also carry moral or religious consequences in civil or religious law.
American researcher Alfred Kinsey found in his 1950-era studies that 50% of American males and 26% of females had extramarital sex.  Depending on studies, it was estimated that 26–50% of men and 21–38% of women, or 22.7% of men and 11.6% of women had extramarital sex. Other authors say that between 20% and 25% Americans had sex with someone other than their spouse. Durex's Global Sex Survey has found that 44% of adults worldwide have had one-night extramarital sex and 22% have had an affair. According to a 2004 United States survey, 16% of married partners have had extramarital sex, nearly twice as many men as women, while an additional 30% have fantasized about extramarital sex. There were also studies that have shown rates of extramarital sex as low as 2.5%.
The Torah prescribes the death penalty through stoning for adultery, which is defined as having sex with a woman who is married to another man. Two witnesses of good character had to testify in court for the case to be even considered by the judges.
Israelite and historic Jewish society was polygynous (one man could have many wives), so the marital status of the man was irrelevant. If a woman, however, is unmarried, a sexual relationship, though highly immoral and sinful from the religion's point of view, is not considered to be adultery, and therefore not punishable by death, but by lashing.
Any physical punishments for any sins were in effect at the times of Judges and the Holy Temple. Now, any physical punishment is prohibited by Judaism—as no proper judicial process can be provided until the Holy Temple is rebuilt by the Messiah.
Extramarital sex is considered to be immoral by Christianity, who base this primarily on passages like 1 Corinthians 6:9-10:
- Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the immoral, nor idolaters, nor adulterers, nor sexual perverts, nor those who practise homosexuality, nor thieves, nor the greedy, nor revilers, nor robbers will inherit the kingdom of God.
It is listed as a sin here and in other passages (along with idolatry, theft, greed, lying, and sexual perversion. While the next verse from the above passage is quick to point out that although some Christians used to practise those sins—that they have since been "justified in the name of the Lord Jesus Christ" and are thus forgiven their sins—extramarital sex has historically been considered to be one of the more serious and damaging sins, possibly because of passages like 1 Corinthians 6:18 that speak of it as sinning against one's own body.
Traditional interpretations of Islamic law (or Sharia) prescribe severe punishments for zina, or extramarital sex, by both men and women. Premarital sex could be punished by up to 100 lashes, while adultery is punishable by stoning. The act of sexual penetration must, however, be attested by at least four male Muslim witnesses of good character, the accused has a right to testify in court, the suspect's word or testimony is required to hold the most weight in the eyes of the judge(s), punishments are reserved to the legal authorities and the law states that false accusations are to be punished severely. The former regulations also make some Muslims believe, that the process's goal was to eventually abolish the physical penalties relating to acts of fornication and adultery that were already present within many societies around the world when Islamic teachings first arose. According to this view, the principles are so rigorous in their search for evidence, that they create the near impossibility of being able to reach a verdict that goes against the suspect in any manner.
Extramarital sex is not illegal in many countries and most states in the United States. Virginia prosecuted John Bushey for adultery in 2001. Other states allow jilted spouses to sue their ex-partners' lovers for alienation of affections.
Extramarital sex is illegal in some Muslim countries, including Saudi Arabia, Pakistan, Afghanistan, Iran, Kuwait, Maldives, Morocco, Oman, Mauritania, United Arab Emirates, Qatar, Sudan, and Yemen.
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