Humanae Vitae is online at the Vatican site, and the text highlighted below is © Copyright – Libreria Editrice Vaticana. For skeptics on HV 11, Pope Paul offers the argument in favor of marital sexual intercourse being both unitive and procreative:
12. This particular doctrine, often expounded by the magisterium of the Church, is based on the inseparable connection, established by God, which (people) on (their) own initiative may not break, between the unitive significance and the procreative significance which are both inherent to the marriage act. The reason is that the fundamental nature of the marriage act, while uniting husband and wife in the closest intimacy, also renders them capable of generating new life—and this as a result of laws written into the actual nature of man and of woman. And if each of these essential qualities, the unitive and the procreative, is preserved, the use of marriage fully retains its sense of true mutual love and its ordination to the supreme responsibility of parenthood to which (people are) called. We believe that our contemporaries are particularly capable of seeing that this teaching is in harmony with human reason.
The stumbling block I’ve heard most often is that the young, fertile married couple is taken for granted as the model on which to base this reasoning. Not every married couple is young, nor are they fertile. For the latter, flaws can emerge which are biological or psychological.
Outside the realm of biology, are married couples of any age or fertility capable of generativity, a wider and possibly more human expression of union?
I would comment on the “supreme responsibility of parenthood.” For the Christian, the supreme loyalty is spreading the Good News. Marriage and bearing children are means to that end. They are not a special “supremacy” different from non-married couples.
And finally, critics of HV are not all capable of seeing “each and every marital act” as harmonious with human reason. Some people don’t get it. Some people deny it. Some people struggle and give up on it. Some adhere in practice and preaching, but reserve skepticism on particulars. In these circumstances, we may well have to rely more on grace and inspiration than mere human reason.