Question:
Answer:
Priests have pastoral discretion in the administration of the sacraments. While dioceses will not forbid a marriage between a man and a woman simply because they’re cohabiting, dioceses require Pre-Cana marriage preparation classes for all couples, and during them will convey to the couples the moral perils of cohabitation, fornication (the two typically go together), and contraception (usually also present with cohabitation and fornication).
Many priests will work with cohabiting couples, showing them the need to make their commitment to each other permanent in the sacrament of marriage and encouraging them to go to confession and refrain from sexual relations until after they’re married.
Other priests will advise a couple that they would be happy to prepare them for marriage and celebrate their wedding, provided the man and woman agree to separate until their marriage. If the couple will not, these priests will wish them well but not proceed with marriage preparation—as a witness to the couple that they need to rethink the serious commitment and related sacrifices of holy matrimony.
Both approaches are legitimate, as both are ordered toward getting couples to enter into marriage as a lifelong, Christ-centered commitment.
For information, read this short answer on why cohabitation is morally wrong.