Evolutionary theory stems from sexual reproduction for the survival of a species. The combination of chromosomes of Deoxyribonucleic Acid (DNA) from male and female organs in plants and male and female animals, allows them to replicate their species. The definition of a species is the ability to create viable fertile offspring. Sexual reproduction is one of the seven characteristics of all living organisms. Sexual reproduction allows the species to survive for a length of time greater than the lifespan of any individual organism. While plants and amoeba can do this asexually, sexual reproduction increases the gene pool and, consequently, the chances of survival. The first mammal not produced by sexual reproduction, or impregnation, was a sheep cloned in 1996. The survival rate of cloned animals has so far been much lower than that of naturally reproduced animals.
Sexual reproduction carries with it a social taboo and ritual in many cultures. The reforming Roman Catholic Church, particularly during the Papacy of Gregory VII, advocated celibacy among monks and nuns and demanded it from the clergy. Prior to that, bishops and even popes with children were not uncommon. The Protestant Calvinist, Lutheran and Church of England schisms rejected the practice of celibacy in the clergy. The idea drew on scriptural references to the profanity of sexual reproduction and traditions such as the miraculous "virgin birth." Other traditions consider reproduction imbued with sanctity, as expressed in the biblical "Song of Songs" attributed to King Solomon.
Many modern Western societies offer incentives such as maternity leave, child benefits and tax relief to families to have children to maintain the society, where there is a fear that families may choose not to have children or restrict themselves to one child, reducing the population size. In contrast, some countries such as India and China have tried to restrict sexual reproduction to reduce the population size, especially by advocating contraception. In some countries, birth control has been prohibited, such as in the United States in the 19th century, where the Comstock Law of 1873 outlawed as obscene the dissemination of contraceptive information and devices. Margaret Sanger (nee Higgins) (1879–1966), a campaigner for women's suffrage, believed that women's equality depended on family planning. She began distributing pamphlets such as Family Limitation and writing newspaper columns in 1912. She founded the American Birth Control League in 1921, presided over the National Committee on Federal Legislation for Birth Control from 1927 and helped organize the first World Population Conference in Geneva. Medically supervised birth control was legalized in many states in 1937.
Fertility treatment assists those unsuccessful in unassisted sexual reproduction. In agriculture and breeding (such as pedigree horses and dogs), artificial insemination has been a common practice. In humans, it is usually restricted to couples where the male or female is infertile.
Recreational sexual activity is believed to be practiced even by some animals, such as dolphins. It is common among humans and advocated as part of a healthy marital bond. In some societies, men, or women, may have more than one spouse, such as in Arabic countries and among the Inuit in Quebec.
The practice of casual sexual relations is sometimes associated with the spread of sexually transmitted diseases. Contraception is advocated to prevent the spread of sexually transmitted diseases such as the human immunodeficiency virus (HIV) that leads to Acquired Immunodeficiency Syndrome (AIDS), as well as hepatitis and syphilis.
In modern Western countries, bigamy is outlawed, although serial monogamy is not restricted. Many religions prohibit sexual relations outside a marital relationship that invokes the mutual responsibility of the couple. Many societies prohibit relations between close relatives as incestuous, including those with relatives by marriage or adoption. The civil rights movement has campaigned against gender discrimination, although some religions prohibit same sex relationships.
Sadomasochistic practices are restricted by modern societies to consenting adults and are prohibited by many religions and societies. These are associated with deviant subcultures. Sigmund Freud identified the libido as the source of sexual desire and a cause of neurosis in patients, usually as a result of suppression of deviant sexual thoughts or experiences. Sexual offenses are considered among the most severe and the most deserving of punishment in modern Western societies, largely based on the absence of consent. Consent cannot be given by a minor, and pedophilia is probably considered the most grievous offense, alongside rape and nonconsensual sexual relations.