If the priest’s parents had a perfect or dysfunctional marriage, there were lessons learned, either on what to do or what not to do. The priest would also have had a circle of friends whose parents had marriages different from his own parents’ that he would have observed over time ... they get as wide a view as most.
And as previously noted, nothing, absolutely nothing, or no body, prepares you for living your marriage day to day, year after year, good times and bad. We should all come into marriage fairly matured and with an open mind and heart, and willingness to adapt, often from day to day.
I agree that a priest has as good a chance of giving an engaged couple or a married couple some useful guidance as anyone else does. My husband and I (married 30 years next February 4) had premarital counseling with a Methodist Air Force chaplain, but a priest or any lay counselor could have done the same “curriculum.”
In my opinion, the main thing about marriage is endurance, and you can be told about that by anyone who’s honest.