Precise definitions vary historically and between and within cultures, but marriage has been an important concept as a socially sanctioned bond in a sexual relationship. Marriage is usually conceived as a male-female relationship designed to produce children and successfully socialize them. Historically, most societies have sanctioned polygamy. The West is a major exception. Europe and the United States were monogamous cultures. This was in part a Germanic cultural tradition, a requirement of Christianity, and a mandate of Roman Law. However, Roman Law supported prostitution, concubinage, sex outside of marriage, homosexual sex, and sexual access to slaves. The Christian West formally banned these practices. Globally, most existing societies do not sanction polygamy as a form of marriage. For example, China shifted from allowing polygamy to supporting only monogamy in the 1953 Marriage act after the Communist revolution. Most African and Islamic societies continue to allow polygamy (around 2.0 billion people). This includes India where polygamy is permitted for Muslim citizens. Probably, less than 3% of all Muslim marriages are polygamous. It is increasingly expensive in an Urban setting, but more useful in rural areas where children are a future source of agricultural labor. Most of the world's population live in societies where polygamy is less common and they are overwhelmingly monogamous. Since the latter decades of the 20th century many of society's assumptions about the nature and purpose of marriage and family have been challenged, in particular by gay rights advocacy groups, who disagree with the notion that marriage should be exclusively heterosexual. Some people also argue that marriage may be an unnecessary legal fiction. This is part of the general disruption of "traditional" families in the West. Since WWII the West has seen a dramatic increase in divorce (6% to over 40% of first marriages), cohabitation without marriage, a growing unmarried population, and children born outside of marriage (5% to over 33% of births), as well as an increase in adultery (8% to over 40%). A system of somewhat serial monogamy has de facto emerged. Still, legally sanctioned non-monogamous marriage arrangements are extremely rare.
In modern times, the term marriage is generally reserved for a state sanctioned union. The phrase legally married can be used to emphasize this point. In the United States there are two methods of receiving state sanction of a marriage: common law marriage and obtaining a marriage license. The vast majority of US states do recognize common law marriage. Many localities do support various types of domestic partnerships.
Since the 12th century, marriage or holy matrimony has been a sacrament in the Catholic Church, as well as other Orthodoxies, where it is defined as a relationship between a man and a woman. The Protestant Reformation reformulated marriage as a life-long covenant. Currently same-sex marriage is legal in some countries and states such as Belgium, the Netherlands, Spain, Canada, and the US state of Massachusetts). This is a natural evolution from the idea of domestic partnerships. Marriage of some kind is found in most societies, and typically married people form a nuclear household, which is often subsequently extended biologically, through children. In the West the nuclear family emerged after 1100. Most non-Western societies have a broader definition of family that includes an extended family network. Alternatively, people may choose to be "childfree". Finally, they may be childless due to infertility, and possibly seek treatment or consider adoption. The term wedlock is a synonym for marriage, and is mainly used in the phrase "out of wedlock" to describe a child born of parents who were not married (see illegitimacy).
In the West marriage has evolved from a life-time covenant that can only be broken by fault or death to a contract that be broken by either party at will. Other shifts in Western marriage since WWI include: (a) Unlike the 19th century women not men get child custody over 80% of the time, (b) both spouses have a formal duty of spousal support (no longer just the husband), (c) Women can vote (i.e., coverture no longer applies),(d) Out-of-Wedlock children have the same rights of support as legitimate children, (e) in most states rape can legally occur within marriage and be punished, (f) husbands may no longer physically discipline/abuse their wife, and (g) property acquired since marriage is not owned by the Title-holder. This property is considered marital and to be divided among the spouses by community property law or equitable distribution via the courts. There is a growing debate about the form(s) that marriage should take. Two of the most hotly-debated variants are discussed below: same-sex marriage and polygamy.