The work titled "Moral Theology" by Saint Alphonsus Liguori contains a few brief passages on marital chastity, that is to say, on which types of sexual acts are permissible in marriage. Of course, all sexual acts are prohibited outside of marriage.
The work in question is ten thick volumes entirely in Latin. There exists no English translation, of which I am aware. The work is a review of opinions by theologians on various moral questions. A question is stated, and then differing answers found among various authors are explained. However, the vast majority of these authors and their cited works are lost in obscurity. The main purpose of the work is to survey opinions of Catholic theologians and authors on various topics in moral theology. So the mere fact that a position is stated in "Moral Theology" does not imply the opinion is sound. Even so, at certain points in the work, Saint Alphonsus Liguori states his own understanding of the topic, rejecting one answer and affirming another.
Book 6, Q. 919
Translation: "Then, if the husband withdraws after climax, but before the climax of the wife, whether it is possible for her to immediately excite herself with touches so as to climax?"
Several cited authors answer "No", saying that it is not licit for the wife to touch herself in this way. "The reason is that the climax of the wife is not necessary to procreation, and also because the climax of the wife, occurring while separated, is not of one flesh with the husband."
Saint Alphonsus then cites some authors who answer the question "Yes", but he rejects their answer saying: "this reason is not persuasive, for if this is permitted to the wife, it ought to be permitted also to the husband." So Saint Alphonsus agrees with the authors who answered "No".
From the above text, we can conclude that the Saint rejected the idea of a wife using any technique, whether touches or any other means, to reach climax, after the husband withdraws. For then the two are not one flesh, and so the unitive meaning is absent from the sexual act. In addition, such a sexual act apart from union is also not procreative. The absence of the unitive and procreative meanings is what makes such a sexual act gravely immoral.
Alphonsus does not accept the idea that the wife has a right to obtain sexual pleasure, regardless of the means, as some modern authors imply. Neither does the Saint treat the subsequent act of the wife and the prior act of natural marital relations as if these together constituted "one act", as other modern authors have claimed.
Moreover, the reasons given by the "No" response, which is the response of the Saint, are completely in accord with the recent teachings of the Magisterium that "each single act", that is to say, "each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life" and that there is an "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." [Pope Paul VI, Humanae Vitae, n. 3, 11, 12]
Book 6, Q. 916
In this question, Saint Alphonsus rejects the proposition, so often asserted today, that the married couple may use unnatural sexual acts, as long as the husband completes the act (i.e. climaxes) only in a subsequent act of natural marital relations.
First, notice that the question asks if this proposal is a mortal sin. As usual, the Saint cites opinions on both sides of the question, first citing some who say "No", it is not a mortal sin, as long as there is no "danger of pollution" (i.e. danger of the husband climaxing during the unnatural sexual act), and others who say that there is no mortal sin in any sexual acts between the spouses. But the Saint rejects these opinions.
Saint Alphonsus says: "But they [other cited authors] affirm, commonly and correctly," that it is a mortal sin. So the Saint states that the correct opinion, which was also the common opinion of the theologians of his day, is that such acts are gravely immoral. The reason he gives is quite compelling:
So Saint Alphonsus Liguori rejected the idea that unnatural sexual acts are moral to use as foreplay, as long as the husband consummates only in a subsequent natural act. And yet this rejected idea continues to be promoted today, mostly by Catholics who hide behind the anonymity of the internet.
Many different excuses are being used today to give approval to the use of unnatural sexual acts within the Sacrament of holy Matrimony. And whether I present the teachings of Saint Augustine and Saint Aquinas (See this article), or of present-day theologians (same article), or the answers given by Saint Alphonsus, I know that some readers will use the flimsiest excuses to dismiss whatever is said, no matter how compelling. But if anyone offers them a way to justify this type of sin, no matter how ridiculous, they adopt it as if it were dogma.
The first principle of ethics is a sincere conscience, which seeks moral truth with severe intellectual honesty. In most discussion on sexual ethics in general and marital ethics in particular, the persons arguing for the approval of illicit acts lack this sincerity. I suppose that many of these promoters of unnatural sexual acts are themselves committing unnatural sexual acts in their marriages. That is why they seek a theological rationalization for their sins, and why they try so hard to convince others to commit the same sins (to make the behavior seem acceptable). And it is true, but very sad to say, that no theological argument carries any weight with them. They do not accept correction for their sins -- neither from the Saints, nor from the Magisterium, nor from Sacred Scripture.
Teachers will have the stricter judgment (cf. James 3:1).
If a married couple unfortunately commit these types of gravely immoral acts, they are responsible before God for their private sins. But when Catholics go online to loudly and repeatedly promote the use of gravely immoral sexual acts within marriage, thereby influencing many marriages (not just their own marriage), the harm is multiplied many times over, and so is the culpability. It is a much graver sin to influence many person to sin mortally, than to commit a single mortal sin yourself. If you sin yourself, you might repent and go to Confession. But if you cause others to sin gravely, and you later repent, how will you undo the damage to those souls?
These false teachers who use the internet to convince many Catholics to sin gravely against the Sacrament of Marriage and against their own bodies will be severely punished by God.
Please take a look at this list of my books and booklets, and see if any topic interests you.