Catholic priests have not always been forbidden marriage. In fact, up to about the 10 century many were married. However several problems arose over married clergy.A major problem was one of inheritance. In a world that had the eldest son inheriting his fathers wealth, the Church saw its lands and even some of its churches fall into the hands of sons who cared little for the Church.
This was a problem because the West had been ruled by Germanic tribes which had certain porperty inheritance views that weakened Papal claims to churches. The East never had those problems being that they continued the system set up by Constantine in which churches and their property belonged to the Church organization and could not be handed down as personal property from father to son.
In fact the in 325, the Council of Nicea, convened by Constantine, rejected a ban on priests marrying requested by Spanish clerics. The Council of Nicea's rulings bind all Christians including those under the Pope's jurisdiction.
The Catholic Church distinguishes between dogma and regulations. The male-only priesthood is Catholic dogma, irreversible by papal decree. The ban on marriage is considered a regulation. As Knight-Ridder put it, "That means the pope could change it overnight if he wished."
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