Question:
Answer:
A man’s having premarital sex does not preclude his becoming a priest. Were it otherwise, St. Augustine of Hippo never would’ve been allowed to be ordained, let alone become a great bishop, saint, and doctor of the Church. Augustine led a hedonistic lifestyle for a good number of years prior to his conversion and once prayed, “Give me chastity, Lord, but not yet,” thereby—along with his subsequent sanctity—qualifying as a good patron saint for many young men today.
If a man is already in priestly formation, i.e., studying at a seminary, and engages in fornication, that would at least presumably lead to his taking a break from his studies to further discern his vocation. It wouldn’t necessarily preclude his ordination, but such a transgression inevitably would give his seminary rector and bishop pause. Some might counter that only his confessor might know. This is true, but I would expect his confessor to advise the seminarian to discuss the matter, given its objective gravity, with his rector.
For related reading, see our tract on Celibacy and the Priesthood.