Pausing, interrupting or prolonging the marital act (can be mortally sinful or non-sinful depending on the intention). It is always a mortal sin if it’s performed with the intention of increasing length or intensity of the sexual pleasure or for making the wife or husband reach climax outside of the natural, normal marital act. It is unnatural to interrupt the sexual act for the sake of mere pleasure. It is also a sign of passion, which is mortal (see St. Thomas in next section). For when a husband or wife engages in acts of unnatural prolonging or interrupting of the marital sexual act, they are nolonger following the primary or secondary purpose of the sexual act (procreation and quenching of lust), but are rather following the motive of satisfying and inflaming their shameful and damnable lust as their (new) primary end or motive during marital relations. That’s why it’s a mortal sin to interrupt the act of marriage for the above mentioned reasons. Further, consider that the Catholic Church teaches that even the normal marital act when performed for the sole sake of pleasure is at least a venial sin, but spouses who are interrupting the marital act for the sake of lust are not even performing the normal and natural marital act, but are hindering or interrupting it. As a consequence, they are committing an action that is inherently sinful and unnatural. Resting or taking pauses however is not sinful whenever the situation demands it. For example, the intercourse could be giving the wife pain or be exhausting the husband who, in sincerity, is trying to finalize the act but cannot do it. All of these and similar examples are not sinful, because they are not performed for the sake of lust. Hence, it is the evil intention of enhancing sexual pleasure while refusing to consummate the marital act in the natural way, by unreasonably interrupting it, or by unreasonably holding on too long, that makes the deed of prolonging marital relations sinful. For everything not following reason in the marital act, as explained by St. Thomas Aquinas in the beginning of this article, is sinful.