Honestly, outside of one woman who threw a major fit that her Catholic priest would not marry her and her fiance if they didn't quit having sex
Thought I should seize the opportunity to add that there is a reason for this
According to the Catholic Church, marriage is a covenant, by which a man and a woman establish between themselves a partnership of the whole of life, is by its nature ordered toward the good of the spouses and the procreation and education of offspring; this covenant between baptized persons has been raised by Christ the Lord to the dignity of a sacrament (From the Catechism of the Catholic) (A sacrament is a visible sign of an invisible grace, namely the divine presence)
Therefore, marriage is not just a ceremony by which two people are legally bound together. As a sacrament, it is an act of worship, and expression of faith, a sign of the Church's unity, a mode of Christ's presence.
And in order to receive the Sacrament fully & worthily, one needs to be free of mortal sins, one of which is pre-marital sex.. That's why the Priest asked this lady to abstain from sex (i.e. to deny the sin) until they're officially married.
(BTW, Mortal sins must meet the following criteria: 1) its subject must be a serious matter. 2) it must be committed with full knowledge, both of the sin and of the gravity of the offense. 3)it must be committed with deliberate and complete consent. Mortal sins cannot be done "accidentally." A person who commits a mortal sin is one who knows that their sin is wrong, but still deliberately commits the sin anyway)
BTW, I don't look to start another discussion about pre-marital sex. We all know where we stand so there is no need to - I just thought I should give the reason why (Catholic) priests are so strict about this.