Patrick J Fletcher, PhD, Senior Theological Advisor, Canadian Conference of Catholic Bishops was kind enough to answer my question.
The sentence you mention, although perhaps unfortunate in its lack of precision, simply intends to express the teaching of the Church (cf. Humanae Vitae 12) that the marital act must have a procreative and unitive significance. It is not trying to say that every marital act must result in procreation, or even have the possibility of resulting in procreation. To the extent that the current wording could imply this, it is unfortunate.
Please be assured, however, that the Pastoral Letter is in no way intending to change the teaching of the Church on sexuality as contained in the magisterium of Pius XI, Paul VI, John Paul II, or Benedict XVI.
Thus, as regards the example you provided of a couple who is sterile for reasons beyond their control (age, disease, etc.), sexual intercourse for them would still possess its procreative significance inasmuch as the essential meaning of the act still speaks the language of life-giving and has not been intentionally thwarted in this regard.
I appreciated being directed to the encyclical letter Humanae Vitae 12 and read with interest much of the letter. This quote from Humanae Vitae section 11 was particularly helpful.
The sexual activity, in which husband and wife are intimately and chastely united with one another, through which human life is transmitted, is, as the recent Council recalled, "noble and worthy.'' It does not, moreover, cease to be legitimate even when, for reasons independent of their will, it is foreseen to be infertile. For its natural adaptation to the expression and strengthening of the union of husband and wife is not thereby suppressed. The fact is, as experience shows, that new life is not the result of each and every act of sexual intercourse. God has wisely ordered laws of nature and the incidence of fertility in such a way that successive births are already naturally spaced through the inherent operation of these laws. The Church, nevertheless, in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches that each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life.
The encyclical letter clearly states that sexual activity between a husband and wife is noble and worthy whether or not the couple is fertile. This is a helpful clarification as it is clear that some Catholics and other readers of this letter have been confused by the language. I encourage readers to take the time to read all of Humanae Vitae and particularly sections 11 and 12. They are a helpful counter-cultural voice to a world focussed on personal gratification.