The Roman Catholic Church teaches that there are three Evangelical Counsels. These are recommendations (counsels) that come directly from the Gospels (evangelical), which are necessary for one consecrated to God to enjoy the fullness of his/her apostolic life. The Evangelical Counsels are: poverty, chastity, and obedience. These three counsels are vowed to God by all consecrated men and women (religious) in the Church.There are two types of priests within the Church; a "religious" priest who vows to live according to the evangelical counsels within the framework of an established religious community, and a "diocesan" priest. The diocesan priest does not live within a religious community, rather he ministers in a specific geographical area. He does not make vows to live the evangelical counsels in the same manner as his religious priest brethren. Upon his ordination, he vows obedience to God, through the Bishop of his chosen diocese. That vow of obedience encompasses a voluntary promise of celibacy for the sake of the Church.
Celibacy for Catholic priests is not a demand, but a voluntary promise made with full and complete knowledge of what said promise entails. Chastity and celibacy are often confused. For the sake of clarification. let's say that while celibacy for Catholic priests indeed includes the practice of chastity, it is not necessarily the same in the essence of its definition. Celibacy is simply to remain unmarried, and thus to abstain from sexual intimacy. Chastity is sexual fidelity to God in one's chosen state of life. Consecrated religious men and women who are asked to vow chastity are generally unmarried, and therefore celibate. There is also the option, lesser known among Catholics and non-Catholics alike, for married men and women in the Church to participate in a sort of consecrated religious life. Because they are married, they are not asked to abstain from sexual intimacy, but they are required to show fidelity to God by participating in said intimacy only with their spouse. Thus, a married man or woman can make a vow of chastity as well as one who is unmarried.
Celibacy for Catholic priests is something in which they are very clearly instructed prior to ordination. They are given liberal instruction in the meaning and value of choosing to remain celibate for the sake of their ministry, and prior to ordination they may choose to move on and pursue a different state of life within the Church. There are many points at which a candidate for the priesthood in the Catholic Church may "turn back" as it were.
However, once the decision is made to accept ordination to the priesthood, the priest becomes then an "alter Christus" - a Latin phrase that means, "another Christ." His spouse, then, becomes the Church - known as the Bride of Christ. His body, then, and his chastity in his chosen state of life requires celibacy. Freely chosen, it allows him to love more deeply and more broadly than would otherwise have been the case.
The Catechism of the Catholic Church describes it thus: "All the ordained ministers of the Latin Church, with the exception of permanent deacons, are normally chosen from among men of faith who live a celibate life and who intend to remain celibate "for the sake of the kingdom of heaven." Called to consecrate themselves with undivided heart to the Lord and to "the affairs of the Lord," they give themselves entirely to God and to men. Celibacy is a sign of this new life to the service of which the Church's minister is consecrated; accepted with a joyous heart celibacy radiantly proclaims the Reign of God." (CCC paragraph 1579).
Mostly, it is a requirement of the Church that a priest remain celibate in order to fully devote himself to the service of God, and to the service of the people who make up the body of the Church to whom he ministers. And, it is something he chooses freely to embrace. It is not, under any circumstances, forced upon him. He voluntarily surrenders his body to God in service to the Church.