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and 5 percent involved two women (U.S. Bureau of the Census, 2005). One-third of heterosexual cohabiting relationships last less than one year. Sixty percent lead to marriage; these marriages are more likely to end in divorce than marriages not preceded by cohabitation (Smith, 2003), although the reasons for this association are not well understood.
Marriage refers to a relationship between two people based on a religious or legal compact. The compact confers recognition and certain rights on partners in an intimate sexual relationship. For centuries, most societies have had established procedures for and recognized marriages involving one man and one woman. Some societies now provide for and recognize marital relationships involving two men or two women, including Belgium, Canada, Netherlands, and Spain. In the United States, a few states allow such marriages, but their legal status is unresolved in 2005. At least 90 percent of the men and women in almost every country in the world marry (United Nations, 2000). Men generally marry at older ages than women throughout the world. Marriage is the social relationship within which sexual expression has the most (in some countries the only) legitimacy. Most married couples engage in sexual intercourse. The frequency of and specific practices that make up sexual expression reflect social norms. In the United States, the frequency of vaginal intercourse within marriage ranges from two times per week among couples ages 18-29 to two times per month among couples ages 60-69 (Smith, 2003). It is likely that a similar decline occurs in most societies. The frequency of sexual intercourse in a long-term relationship reflects both biological (changes associated with aging, illness) and social (habituation to partner, quality of the relationship) factors. There is wide variation in frequency; there are young couples who never engage in intercourse, and older couples who engage in it several times per week. Other forms of sexual expression also occur in marriage, including behaviors such as oral-genital sexuality, anal intercourse, and practices such