This point will be discussed at length in my upcoming book. But here is the text from the Address to the Second World Congress on Fertility and Sterility of Pope Pius XII:
“By the force of this law of nature, the human person does not possess the right and power to the full exercise of the sexual faculty, directly intended, except when he performs the conjugal act according to the norms defined and imposed by nature itself. Outside of this natural act, it is not even given within the matrimonial right itself to enjoy this sexual faculty fully. These are the limits to the particular right of which we are speaking, and they circumscribe its use according to nature.”
“What has been said up to this point concerning the intrinsic evil of any full use of the generative power outside the natural conjugal act applies in the same way when the acts are of married persons or of unmarried persons, whether the full exercise of the genital organs is done by the man or the woman, or by both parties acting together; whether it is done by manual touches or by the interruption of the conjugal act; for this is always an act contrary to nature and intrinsically evil.” 
This teaching absolutely condemns as intrinsically evil and gravely immoral the idea — promoted loudly and incessantly by many Catholic commentators — that the wife may climax before or after the natural marital act, by the use of unnatural sexual acts to completion. The holy Pontiff teaches that neither the husband nor the wife may deliberately choose (directly intend) the “full exercise of the sexual faculty” outside of the natural act. No matter by what means this is accomplished (“by manual touches” or anything else), it is “always an act contrary to nature”, meaning an unnatural sexual act, and is “intrinsically evil”.
Pope Pius XII does not propose this idea as his own opinion, but as the “law of nature”. And he does not say that this sin is due to a bad intention, or a certain circumstance. Instead, any such sexual act is “intrinsically evil”. But we know from the teaching of Pope Saint John Paul II in Veritatis Splendor that intrinsically evil acts are never justified by a good purpose (such as the purpose of preparing for the natural marital act), nor by a difficult circumstance (such as that the wife cannot reach climax in the natural act).
The claim made by West and Popcak and others that the wife may reach climax outside of the natural marital act, supposedly because her climax is not related to procreation, is contrary to the teaching of Pope Pius XII and contrary to the natural law. It doesn’t matter “whether the full exercise of the genital organs is done by the man or the woman, or by both parties acting together,” it is intrinsically evil and always gravely immoral to deliberately choose a sexual act to completion, other than the natural marital act.
Pope Pius XII makes no exception for unnatural sexual acts done shortly before or after the natural marital act. And he does not accept the ridiculous idea that all the sexual acts of one session in the marital bedroom are “one act”. Rather, the natural marital act is one sexual act, and anything done before or after is a separate act.
This teaching of Pope Pius XII reveals the claims of many commentators on this topic to be plainly erroneous. A set of sexual acts in the marital bedroom are not one act. The wife may not deliberately climax outside of the natural marital act. The end of natural marital relations does not justify the means used to reach that end, such as completed unnatural sexual acts on the wife. All these things are incompatible with the teaching stated above.
Can we infer from his prohibition of sexual acts to completion, outside the natural marital act, that incomplete unnatural sexual acts are permitted? No, we cannot. And this point will become clear in the further explanations that I give in my next book, which is on this subject. See my previous post: Church Teaching on Unnatural Acts in Marriage and the article on the teaching of St. Alphonsus Liguori.
How do certain married Catholics expect to be welcomed into Heaven, when they abort their own children by using abortifacient contraception, and they commit innumerable unnatural sexual acts within the Sacrament of holy Matrimony. Then they receive Communion, without prior repentance and confession. All these grave sins will be judged by God.
More on marital chastity in The Catholic Marriage Bed
Please take a look at this list of my books and booklets, and see if any topic interests you.
 Pope Pius XII, Address to the Second World Congress on Fertility and Sterility, 19 May 1956; original languages are French and Latin. Translation here, from the Latin section of the Address, is by the author, Ronald L. Conte Jr.