Strictly speaking sacerdotal celibacy is the requirement of continence (refraining from any form of sexual intercourse, not the other one!) in the Latin Church, celibacy refers to a renouncing of marriage. Since in marriage you are obliged to consummate and reproduce then the observation of continent behaviour is the overriding rule. The following canon refers:
Clerics are obliged to observe perfect and perpetual continence for the sake of the kingdom of heaven and therefore are bound to celibacy which is a special gift of God by which sacred ministers can adhere more easily to Christ with an undivided heart and are able to dedicate themselves more freely to the service of God and humanity.
So as wizard has quite aptly said really. They make a vow of chastity � to remain pure and conform to sexual morality. It wasn�t always the case of course. In the early church most priests and monks were married and had lovers. But because of their vows, the clergy were expected to live in complete continence, refraining permanently from sexual relations with their wives and because of the exclusion of sexual relations, the members of the clergy were not entitled to marry. Although it was in Canon Law, it wasn�t enforced, but by the 3rd C, penalties were introduced for those clergy (and their wives) not living in accordance with their vows.
It took a long time for the Canon Law to become de rigueur for the clergy, even to the point of the children of clergy being declared illegitimate and the wives being sent off to convents. It wasn�t really until the First Lateran Council in 1123 that clerical marriage and conjugal relations was forbidden, this was reinforced again in the Second Lateran Council 16 years later.
My aunt is a 74 year old Roman Catholic Nun, and I do think it is a great shame that she had a relationship or a family, such is her devotion to her vows.