Christian teaching, and the Natural Law, had already been proclaimed many times by previous popes. They were reaffirmed by his predecessor, and at Vatican II, after
the oral contraceptive had been developed. It is reasonable that Pope Paul VI expected those reaffirmations (the ink was still damp) to have been read.
Pope Pius IX promulgated the encyclical Casti Connubii in 1930, reaffirming Catholic teaching on artificial birth control. During Vatican II, ABC was again discussed, and Pope Paul VI appointed a commission to study the question. The commission recommended a change in Catholic doctrine. The pope set the question aside, and nothing more was done during the council. Many anticipated a change. That the doctrine could be changed was not even a question.
It was not until 1968 when Pope Paul VI promulgated the encyclical
Humanae Vitae. It created an uproar within the Church, with even some Bishops saying the encyclical could be ignored. The several sources I’ve read all say the same thing: Pope Paul VI was reluctant to change the doctrine about ABC, fearing that doing so would set a precedent and weaken papal authority for future popes. It has also been argued that it was
Humanae Vitae, and not Vatican II itself, that resulted in the major crisis in the Church following Vatican II. A number of Catholic theologians were beside themselves, having both anticipated and predicted a change in the doctrine. There were major consequences for it.
During Vatican I, Pope Pius IX essentially declared that he (and future popes) were infallible with respect to the teaching of faith and morals. At that time, there was also an uproar. Cardinal Newman was one of many prominent Catholics who strongly opposed this doctrine, and he did so until he was silenced. There is an ancient Church teaching known as the Doctrine of Reception. It provides that for a doctrine to be legitimate, it must be accepted by the faithful. This obviously could have been an issue with respect to
Humae Vitae, but this Catholic doctrine had died a slow death over the centuries.
The Dogmatic Constitution
Dei Verbum was in part a response to what was by Vatican II perceived by many as a difficulty in that it gave sole and infallible authority, in perpetuity, in these matters to one person. With the promulgation of
Dei Verbum, doctrine and papal authority were no longer invincible. However, for whatever reasons, this dogma is scarcely mentioned.