The following passages are quoted from the revised edition of my book The Catholic Marriage Bed, available at Amazon in Kindle Format and as a Paperback Book.

Chapter 1 presents the teaching of the ordinary and universal Magisterium on the basic principles of ethics:
* that morality is based on human acts, which are deliberate knowing choices,
* that each deliberate knowing choice is subject to the eternal moral law,
* that each act stands on its own, as to its morality, based on the three fonts (intention, object, circumstances),
* and that intrinsically evil acts are never moral.

The chapter concludes:

The application of the above basic principles of ethics to the marital bedroom is not complicated. Each knowing deliberate choice is an act, subject to the eternal moral law. An act cannot borrow the fonts of a different act. If you rob a bank, in order to pay off your debts, the good act of paying off your debts does not justify the intrinsically evil act of robbery. You cannot claim that the act of bank robbery has the moral object of the act of paying off your debts. Each act must be evaluated based on its own fonts. A set of sexual acts is not “one act”, nor can a non-procreative sexual act borrow the procreative finality of an act of natural marital relations so as to have a good moral object (procreation) in place of its own evil moral object of the deprivation of the procreative meaning.

Unnatural sexual acts are inherently non-procreative and non-unitive. Then, when climax is lacking, these acts remain intrinsically evil. For the lack of climax does not remedy the deprivation of the unitive and procreative meanings. Sexual acts are intrinsically evil whenever they have an evil moral object. The deprivation of any one or more of the marital, unitive, and procreative meanings makes that sexual act intrinsically evil. And the commission of a good act of natural marital relations about the same time does not change the morality of that intrinsically evil act.

Sometimes, poorly-catechized Catholics try to justify a sinful act by misapplying the principle of double effect. But the first rule of that principle is the act in question must not be intrinsically evil. So the very first step, to apply the principle of double effect, is to consider the moral object of the act. If the object is evil, then the act is intrinsically evil, and it cannot be justified by the principle of double effect. Unnatural sexual acts are intrinsically evil, due to the deprivation of the unitive and procreative meanings, so the principle of double effect does not justify those acts.

A good intended end (or purpose), such as to prepare for the natural marital act, does not justify the commission of an intrinsically evil sexual act (or any intrinsically evil act). Therefore, the purpose of foreplay does not make the acts used as foreplay necessarily automatically moral. Any act used as foreplay must be moral based on the three fonts of morality that spring up from, and apply to, that act.

The end does not justify the means. Therefore, the end of performing natural marital relations does not justify all that is done to achieve that end. Only moral means may be used to attain moral ends. Moral acts of ordinary foreplay may be used to prepare for the natural marital act. Unnatural sexual acts may not be used, as these acts are inherently non-procreative and non-unitive, and therefore intrinsically evil.

The claim that there is only “One Rule” in the marital bedroom, that the husband climax in the natural manner, is absurd. It is patently a rejection of all of the basic principles of ethics. The Church teaches that each act, that is to say, each knowing deliberate choice, must be good — based on the intention, the circumstances, and the moral nature of act (as determined by the object). If the “One Rule” were correct, then all manner of intrinsically evil acts would be justified, in addition to bad intentions and harmful circumstances. Such a blatant abandonment of all moral teachings should cause faithful Catholic Christians to recoil in disgust.

But since the real purpose of the “One Rule” is to justify every kind of pleasurable but intrinsically evil sexual sin, very many persons have made that Rule their banner. They trumpet this idea across the internet. They explicitly refer to this rule to justify sodomy, masturbation, oral sex, and all manner of pornographic acts in the marital bedroom. And they have the gall to call such acts “holy sex”. The word holy refers first and foremost to God, and then secondly to things close to God, such as the Sacraments and the Saints. To call perverse and degrading behavior “holy sex” is to commit indirect blasphemy against God, who alone is Holy.

RLCJ