Humanae Vitae is twaddle. It is based on a belief, promulgated by unmarried men, to whom sex was evil (gnostic dualism is heretical, but it still lives on in Catholic moral theology) and had to be "justified". The way to justify sexual intercourse was say that it was to be used for procreation. For example, Augustine's "On the Good of Marriage" says that any married couple who have sex without the express intention of procreating was sinning.

Here is the heart of HV's argument, from section 4: "This kind of question requires from the teaching authority of the Church a new and deeper reflection on the principles of the moral teaching on marriage—a teaching which is based on the natural law as illuminated and enriched by divine Revelation. No member of the faithful could possibly deny that the Church is competent in her magisterium to interpret the natural moral law". That says, "We know it's true. How do we know? Because we say so." Sorry, but I've given up circular reasoning for Lent.