There is a good briefing on this question here. Some selections:
From the perception that Catholic priests are all celibate, the idea has spread that clerical celibacy consists in the prohibition of marriage. And therefore that "moving past" celibacy consists both in ordaining married men as priests and allowing them to continue living conjugal life, and in permitting celibate priests to marry. After Vatican Council II, both of these requests have been advanced repeatedly in the Catholic Church, even by bishops and cardinals. But both of them are in clear contrast with the entire tradition of this Church itself, beginning from the apostolic era, and in the case of the second request with the tradition of the Eastern Churches as well, and therefore with the journey of ecumenism.
The fact that since the beginning of the Church priests and bishops were required to abstain from conjugal life is confirmed by the first rules written on the matter. These began to appear in the fourth century, after the end of the persecutions. With the sharp rise in the number of the faithful, ordinations also increased, and with them the violations of continence. Against these infractions, councils and popes intervened repeatedly to reaffirm the discipline they themselves called "traditional." This was done by the Council of Elvira in the first decade of the fourth century, which punished lack of respect for continence with exclusion from the clergy; other councils a century later; popes Siricius and Innocence I; and still other popes and Fathers of the Church, from Leo the Great to Gregory the Great, from Ambrose to Augustine to Jerome.
For many more centuries, the Western Church continued to ordain married men, but always demanded that they renounce conjugal life and separate from their wives, after receiving their consent. Infractions were punished, but they were very frequent and widespread. In part to combat this, the Church started trying to select its priests from among the celibate.
In the East, however, from the end of the seventh century onward the Church held firm the absolute obligation of continence only for bishops, who were increasingly chosen from among monks rather than from among married men. With the lower clergy, it allowed the married to continue leading a conjugal life, with the obligation of continence only "on the days of service at the altar and of the celebration of the sacred mysteries." This was established by the Second Council of Trullo in 691, a council never recognized as ecumenical by the Western Church. From then until now, this is the discipline that has been in effect in the East, as also in the Churches of the Eastern rite that have returned to communion with the Church of Rome since the schism of 1054: absolute continence for bishops, and conjugal life permitted for the lower clergy. On the condition that marriage must always precede sacred ordination, and never follow it.
IN ORDER TO UNDERSTAND THIS WE NEED A POSITIVE APPRECIATION OF CELIBACY. See John Paul II's Theology of the Body, especially sections 73 onwards. I'll try to post other helpful pieces in due course.