Ryan McAvoy said:

I'd be interested to know which Pope brought in mandatory Celebacy, when it was and what were his reasons?

The rule of celibacy was recognized in the west (i.e. in Europe) by the time of Pope St. Leo I (the Great) who was Pope from AD 440-461. It was made a rule in the Synod of Elvira (AD 305 (as a synod it is and was not applicable to the church as a whole but directed at a certain region) and reaffirmed in the Council of Carthage (AD 390). The rule of celibacy is not a Church doctrine, meaning that it is a rule that is not universal in the Church and can be changed. It is what is known as a discipline and its purpose is to allow a priest to focus on God and his parish alone, without the difficulties of having a spouse and a family. In most rites of the Catholic Church, the Latin Rite being the main exception, priests are allowed to be married when they are ordained but cannot marry afterwards. In the Latin Rite, that same rule applies to deacons only. Married converts to the Catholic Faith can be granted exception to the rule of celibacy in priesthood and can be ordained. This usually happens with Protestant ministers who convert to the Faith and in fact this happened recently in my diocese.

The rule of celibacy might change in the future, or more allowances may be made. It is understandably a frequently misunderstood rule and one that causes a lot of criticism of Catholics, but hopefully my explanation makes the rule a bit clearer to you (if not, feel free to clarify if you wish).