I think that was probably a big part of it. When the Episcopalians, ECLA Lutherans, and other “mainline” Protestants were ditching traditional moral teachings, some people thought the Catholic Church was jumping on the same bandwagon. By the time Paul VI realized what was going on and issued Humanae Vitae, the “Spirit of Vatican II” had become a runaway train.
In addition to the timing, the Vatican II documents were written in an “academic” manner that, while containing thorough scholarly analysis, can easily be taken out of context and twisted to suit a contrary agenda. In other words, it took teachings that were presented previously as “black and white,” and re-presented them as “grey.”
**Finally, the Vatican expected too much from people. I believe they actually thought that, if something were made optional rather than mandatory, or alternatives offered, but with the reason for it thoroughly explained, people would still do it. ** In fact, people take the path of least resistance and, if they don’t have to do it, many won’t. Abstinence of meat on Fridays is a perfect example: The Church says, after explaining the reason for the practice in the first place, “therefore, you don’t have to abstain on meat on Fridays if you substitute another act of penance,” but all people hear is “you don’t have to abstain from meat on Fridays.”