One was not admitted to the seminary without a very very rigorous approval first from the candidate's pastor, other letters of recommendation, schooling etc. even before going before the seminary screening committee.
That does not mean to say that bad apples did not slip through on occasion, but they would be caught somewhere along the line.
The problem happens when one does slip through then gets promoted and then always tends to bring in other bad fruit as itself.
Bella Dodd, when she renounced Communism and returned to the Church, testified in the early 1950's before the U.S. House of Representatives Un-American Activities Committee that the group she headed, by itself, was responsible for infiltrating 1100 Communists into Roman Catholic seminaries in the United States in the 1930's alone. She said that by that time they had mostly all advanced in rank and were monsignors and bishops. The key positions they strive for are those of gatekeeper for admittance of new candidates to the clergy and the convent in order to bring BAD candidates in and even more importantly for their agenda, to keep good candidates out.
If Communists working for THEIR OWN puppet masters who hide behind the scenes, seek to destroy the Church, the surest way besides their own efforts is to totally overrun it with swarms of termites, the homosexuals.
In modern Vatican-2 practice, in almost all dioceses candidates to the seminary must go before some radical lesbian nun, no lie, at the diocesan headquarters and unless they at least 'tolerate' homosexuality and are in favor of women's ordination, despise anything traditional, HAVE NO PERSONAL PRAYER LIFE, and are at least socialist if not worse, they are ABSOLUTELY refused admittance.
Two good books written on this subject are by 'conservative' Novus ordo authors, "Goodbye Good Men" by Michael Rose and "The Rite of Sodomy" by Randy Engel. They will give you chapter and verse examples of EXACTLY what P have stated above.
St. Peter Damian, Doctor of the Church who died in 1072 was a great teacher and preacher in many areas, but is well known for his preaching against unchasteness and sodomy especially.
St. Alphonsus de Ligouri doctor of the Church quoting st. Peter Damian:
St. Alphonsus on the damnation of the impure: “Continue, O fool, says St. Peter Damian (speaking to the unchaste), continue to gratify the flesh; for the day will come in which thy impurities will become as pitch in thy entrails, to increase and aggravate the torments of the flame which will burn thee in Hell: 'The day will come, yea rather the night, when thy lust shall be turned into pitch, to feed in thy bowels the everlasting fire.” (Preparation for Death, abridged version, p. 117)