Monastic orders of many religious traditions have rules concerning celibacy. Marriage and sexual union are forbidden for practical or spiritual reasons.
Sometimes, as in some Hindu and Buddhist traditions, sexual expression is considered to be a detriment to meditation and growth, a "giving in" to the body and its desires. Celibacy is then considered to be a form of asceticism.
In early Christianity, celibacy was inspired by the words of the apostle Paul advising that those who chose not to marry had more time to serve the Lord. He implied that sex, as opposed to spiritual work, was a base human need when he said it "was better to marry than to burn." And since the early church believed Jesus was soon to return, it didn't make much sense to settle into a stable home life that would not last much longer. Besides this, Jesus had said that "in heaven they neither marry nor are given in marriage." All these together seemed to imply marriage was for the weak, at best.
This notion was taken to its logical conclusion by the Shaker community in the United States. Nobody was allowed to have sex. The community only grew by conversions.
In Roman Catholic tradition, celibacy is seen as a crucial vow taken by ordained clergy to allow them both time and uninterrupted energy to devote to the practical matter of being available for ministry. It is not that clergy are not married. They are married to the church and are expected to give the same devotion to Christ as they would to earthly spouses. Although a mystique surrounding "unavailable" male priests and "pure" female nuns undoubtedly places celibate clergy on a pedestal in the popular psyche, this was not the official intention of the church.
the obligatory unmarried state of the Catholic clergy. The decrees of the popes on celibacy in the early Middle Ages were not in fact adhered to. Strict observance of celibacy was demanded by Pope Gregory VII (1073–85), a leader of the Cluniac reform, who forbade married priests to perform their duties. Celibacy became firmly established in practice in the mid-13th century. The Catholic Church used it as a means of preserving church landownership because it prevented land from being broken up among heirs. Attempts were made to review the question of celibacy at the Second Vatican Council (1962–65). Pope Paul VI cut off discussion of the topic, but allowed deacons, including married deacons, to perform certain priestly functions. In 1967 the pope reaffirmed the irrevocability and “sanctity” of celibacy.
The Protestant churches reject celibacy. In the Orthodox Church, celibacy is obligatory only for monks.