The practice of celibacy in the priesthood is apparent in the years following Jesus’ resurrection. Single priests and priests who were married abstained from sex, of course with approval from their wives. Just as Jesus chose celibacy giving up a family in order to give himself to mankind, priests are called by God to imitate Jesus. In fact, the priest is able to better serve all people because he is more available.
Monsignor Angelo Amato of the Prefect of the Congregation of the Causes of Saints states:
“Jesus was chaste, virgin, celibate and he defended it. His virginity distanced him from others, but it’s what made him able to show, compassion and forgiveness to others.”
Thus priests are called by God to imitate Jesus in this discipline.
By the end of the fourth century Pope Saint Siricius pushed for a celibate priesthood in order to maintain continuity with earlier centuries. Later this became a discipline* in order to carry out the tradition of celibacy, thus priests could not marry in the Catholic Church.
* The Eastern Orthodox still allow their priests to marry, but they must be so before entering the seminary and are not allowed to become bishops.