The only moral sexual act is natural marital relations open to life. The Magisterium of the Catholic Church teaches that each and every sexual act must be marital and unitive and procreative.
Pope Paul VI: "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. This particular doctrine, often expounded by the magisterium of the Church, is based on the inseparable connection, established by God, which man on his 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 man is called." (Humanae Vitae, n. 11-12).
Only natural marital relations (natural genital-to-genital intercourse) open to life has all three meanings: marital, unitive, and procreative. If a husband or wife are infertile, due to old age, or injury, or illness, the natural marital act remains moral because it is still the type of act which is inherently directed toward procreation (even if procreation is not attained). The essential moral nature of any act is determined by its inherent ordering toward its moral object, not by the attainment of the moral object.