The U.S. Conference of Catholic Bishops (USCCB) teaches that all sexual acts outside of marriage are gravely immoral, and that within marriage each and every sexual act must be unitive and procreative (open to life). The official Catechism of the USCCB, titled "U.S. Catholic Catechism for Adults," condemns every type of sexual act outside of marriage, including masturbation, premarital sex, homosexual acts, and adultery. All non-marital sexual acts are gravely immoral [p. 406-407, 410]. Then the USCCB Catechism teaches on sex within marriage:
"Each and every sexual act in a marriage needs to be open to the possibility of conceiving a child." [p. 409]
It is not sufficient for some acts within a set of sexual acts to be unitive and procreative. Each and every sexual act in a marriage must have both meanings, the unitive and the procreative. To be moral, a sexual act must be marital, unitive, and procreative.
A pastoral letter from the U.S. Bishops, Marriage: Love and Life in the Divine Plan, elaborates further:
"Sometimes one hears it said that as long as the marriage as a whole is open to children, each individual act of intercourse need not be. In fact, however, a marriage is only as open to procreation as each act of intercourse is, because the whole meaning of marriage is present and signified in each marital act. Each marital act signifies, embodies, and renews the original and enduring marital covenant between husband and wife. That is what makes intercourse exclusively a marital act. "
In other words, moral sexual intercourse only occurs in marriage, and each single marital act (sexual act in a marriage) must be unitive and procreative.
This teaching by the USCCB, which is also the teaching of Humanae Vitae, necessarily implies that every unnatural sexual act is gravely immoral within marriage (and also outside of marriage, of course). For unnatural sexual acts are unnatural precisely because they are inherently deprived of the procreative meaning. Now some unnatural sexual acts have some type of mere physical union, but the unitive meaning is more than physical unity. Unnatural sexual acts are not the type of union ordained by God for a husband and wife. Only the natural sexual act is truly and fully unitive, in the theological and moral sense of the word.
What happens if the married couple are not capable of procreation due to injury, illness, or old age? The pastoral latter comments:
"It is true that some marriages will not result in procreation due to infertility, even though the couple is capable of the natural act by which procreation takes place. "
The act of natural marital relations remains moral, because it is open to life. In other words, the natural type of sexual act retains its good ordering toward the procreative meaning, even when that procreative end is not attained. Moral acts must be inherently ordered toward only good ends. And notice that the USCCB refers to marital relations open to life as "the natural act", implying that other types of sexual acts are unnatural. They are unnatural because they are contrary to the natural law.
Now some commentators claim that a husband can commit unnatural sexual acts in marriage, as long as he delays climax until a subsequent act of natural marital relations. They claim that such acts are not gravely immoral or are not really sexual acts due to the absence of climax. But the USCCB, in their recent document on pornography (Create In Me A Clean Heart) rejects the idea that an act must include climax to be a gravely immoral sexual act.
"Masturbation, which is deliberate, erotic stimulation often to the point of orgasm, commonly occurs together with pornography use. While popular culture largely sees it as acceptable, masturbation is always gravely contrary to chastity and the dignity of one’s body." [III. Pornography's link to other sins]
Notice that the grave sin of masturbation remains intrinsically evil and gravely immoral even when climax is absent. The text says "often" to the point of climax because climax is not essential to the definition of any grave sexual sin. Saint Alphonsus Liguori makes this same point. See my previous post: Saint Alphonsus Liguori on Marital Chastity.
Unnatural sexual acts are intrinsically evil and always gravely immoral, even in marriage, even when climax is absent, even when these acts are accompanied by an act of natural marital relations open to life.
Please take a look at this list of my books and booklets, and see if any topic interests you.