Pius XI gave the definitive judgment in Casti Connubii 56 just a few decades earlier, which of course was no different than what the Church had ever taught. My understanding is that the debate leading up to Humanae Vitae was not centered on contraception generally, but The Pill specifically and whether it violated the principles the Church had always held to (of course, this narrow question was conflated in many minds). This is why Vatican II, in Gaudium et Spes, provided a general condemnation of methods of birth control contrary to divine law citing Casti Connubii, but in the footnotes noted that certain questions were being studied, and the Council left it to the Pope to give the definitive judgment on those.
The argument for The Pill was that there was no spilling of the seed or mutilation that caused sterility or abortion (we know different now), and was simply extending the natural infertile period.
I’m not sure you’re right as to what the “majority opinion” of theologians at the time was (there was definitely not a diachronic majority!)–but it is generally understood that it was the majority opinion of the committee the Pope commissioned to study the issue. But of course, the Holy Spirit was not promised to any commission, but to the Roman Pontiff.
Similarly, St. Peter didn’t side with the majority opinion either when Christ asked him who He was (the majority said John the Baptist, Elijah, or one of the prophets). This is why we have the office of Pope.
Also, just to add, I think the rationale is clear and understandable. I think the issue is that people think it is too hard to live up to and is unreasonable in that sense (as opposed to being logically unreasonable). This was the case even back in Pius XI’s days (he addresses it in Casti Connubii in the paragraphs after his definitive judgment).