A common critique of theistic religion is the extent to which religious rules and doctrines created by human beings for the purpose of maintaining power and control over others are attributed to a divine source. Pretending that human rules are God's rules help prevent them from changing or being questioned. A strong example of this is the celibacy of priests in Catholic Christianity, as demonstrated by its historical development and lack of consistent adherence.
If there were any divine origin to religious rules, we shouldn't be able to trace their development in human history and how they were conditioned by historical, cultural circumstances. It's no surprise that churches say little about how today's doctrines did not always exist in the past and, in fact, aren't as absolute as they seem. Again, clerical celibacy in Catholicism is a good example of this.
Real Reasons for Celibacy: Land, Purity, Women
Celibacy has not always been required of priests. Defenders of celibacy rely heavily on Matthew 19:12, where Jesus is quoted as saying that "...they have made themselves eunuchs for the sake of the kingdom of heaven. Whoever can accept this ought to accept this." Here, "eunuchs" is interpreted to be a reference renouncing marriage and being celibate, but if Jesus placed such a high value on celibacy, why were most if not all of his apostles married? It's implausible that unmarried followers could not be found, so it's implausible that celibacy was required.
Over time, rules about sexual abstinence grew from a belief that sexual intercourse makes a person "unclean," based largely on the belief that women are less pure than men and hence constitute a form of ritual contamination. Attitudes about ritual cleanliness have played an important role in religious violence generally; attitudes about the inferiority of women have been important in violence towards them. In fact, the continued existence of an all-male, celibate priesthood cannot be divorced from an accompanying view of women as less moral and less worthy than men.
The denigration of both women and sex was accompanied by a denigration of marriage and family. The Council of Trent, called to combat the challenges posed by the Protestant Reformation, made an interesting statement about the church's position on family values:
If anyone says that it is not better and more godly to live in virginity or in the unmarried state than to marry, let him be anathema.
Another factor in the push for clerical celibacy was the problematic relationship the Catholic Church had with real estate and inherited land. Priests and bishops were not just religious leaders, they also had political power based on the land they controlled. When they died, the land might go to church or the man's heirs — and naturally the church wanted to keep the land in order to retain political power.
Tthe best way to keep the land was to ensure that no rivals could claim it; keeping the clergy celibate and unmarried was the easiest way to accomplish this. Making celibacy a religious obligation was also the best way to ensure that the clergy obeyed. Catholic apologists deny that such worldly concerns were part of the decision to impose celibacy on priests, but it can't be a coincidence that the final push towards celibacy occurred when conflict over land were increasing.
Evolution of Rules on Celibacy
Because of the doctrine that sexual intercourse with a woman makes a man unclean, married priests were prohibited from celebrating the Eucharist for a full day after sex with their wives. Because the trend was to celebrate the Eucharist more and more often, sometimes even daily, priests were pressured to be celibate just to fulfill their basic religious functions — and eventually they were prohibited from ever having sex with their wives. Celibacy was thus somewhat common by 300 CE, when the Spanish Council of Elvira required married bishops, priests, and deacons to permanently abstain from sex with their wives. The pressure this put on marriages was not important and the consequences for the wives would only get worse.
In 1139, the Second Lateran Council officially imposed mandatory celibacy on all priests. Every priest's marriage was declared invalid and every married priest had to separate from their wives — leaving them to whatever fate God had in store for them, even if it meant leaving them destitute. Of course this was an immoral thing to do to those spouses, and many clergy realized that there was little religious or traditional basis for it, so they defied that order and continued in their marriages.
The final blow against priests' ability to marry came through a technicality at the Council of Trent (1545-1563). The church asserted that a valid Christian marriage must be performed by a valid priest and in front of two witnesses. Previously, private marriages performed by priests or, indeed, just about anyone else, were common in some areas. Sometimes the only ones present were the officiant and the couple. Banning such clandestine marriages effectively eliminated marriage for the clergy.
Contrary to what defenders might say, there is nothing whatsoever about the nature of the priesthood which makes celibacy necessary or essential. In the 1967 encyclical Sacerdotalis Caelibatus, written to reinforce the "Sacredness of Celibacy" in the face of growing calls to rethink it, Pope Paul VI explained that while celibacy is a "dazzling jewel," it is not:
...required by the nature of the priesthood itself. This is clear from the practice of the early church itself and the traditions of the Eastern churches.
The history of clerical celibacy in the Roman Catholic Church is thus one of contingency and political expediency. The doctrine of sexual abstinence, supposedly designed to increase priests' purity, is inseparable from the political and worldly concerns of Christianity at a particular time and place in history. That is also why there are so many married Roman Catholic priests in the world.
More: Married Catholic Priests