Clerical Celibacy: Concept and Method | Alfons Maria Cardinal Stickler | From The Case for Clerical Celibacy
1. The first and most important prerequisite for a knowledge of the historical development of any institution is the proper understanding of the meaning of the concepts on which it is based. For ecclesiastical celibacy, we have a particularly clear and concise reference in the writings of one of the greatest of the Decretists--commentators on Gratian's Decretum--who around 1140 collected and explained all the material concerning the juridical tradition of the first millennium of the Church. This Decretist is Huguccio of Pisa (d. 1210), who in his Summa on the Decretum, composed around 1190, began his treatment of celibacy with these words: "In hac Distinctione incipit (Gratianus) tractare specialiter de continentia clericorum, scilicet quam debent observare in non contrahendo martimonio et in noti utendo contracto." 
A reading of this text clearly indicates a double obligation with respect to celibacy: not to marry and, if previously married, not to use the rights of marriage. In addition, it is clear that even in this period, namely, the end of the twelfth century, there were clerics in major orders who had been married prior to ordination. In fact we know from the Scriptures that the ordination of married men was a normal enough event.