The only moral sexual act is natural marital relations open to life. To be moral, each and every sexual act must be marital and unitive and procreative. This threefold meaning of the marital act is the moral object of sexual relations. If any one or more of these three meanings is absent in any sexual act, then the moral object is evil and the sexual act is intrinsically evil and always gravely immoral.
Unnatural sexual acts are non-procreative, intrinsically evil, and always gravely immoral, regardless of intention or circumstances, even within marriage. Unnatural sexual acts cannot be justified as a type of foreplay in order to prepare for the natural marital act because the end never justifies the means. And the absence of sexual climax does not change an intrinsically evil, gravely immoral, unnatural sexual act into an act that is good or morally defensible.
“…nor be changed into that use which is against nature, on which the Apostle could not be silent, when speaking of the excessive corruptions of unclean and impious men… by changing the natural use into that which is against nature, which is more damnable when it is done in the case of husband or wife.” (Augustine, On the Good of Marriage, section 11).
“For, whereas that natural use, when it pass beyond the compact of marriage, that is, beyond the necessity of begetting, is pardonable in the case of a wife, damnable in the case of an harlot; that which is against nature is execrable when done in the case of an harlot, but more execrable in the case of a wife… But, when the man shall wish to use the member of the wife not allowed for this purpose, the wife is more shameful, if she suffer it to take place in her own case, than if in the case of another woman.” (Augustine, On the Good of Marriage, section 12).
“And since the man who is too ardent a lover of his wife acts counter to the good of marriage if he use her indecently, although he be not unfaithful, he may in a sense be called an adulterer; and even more so than he that is too ardent a lover of another woman.” (Saint Thomas Aquinas, Summa Theologica, II-II, Q. 154, article 8).
“Lastly comes the sin of not observing the right manner of copulation, which is more grievous if the abuse regards the ‘vas’ [vessel, orifice] than if it affects the manner of copulation in respect of other circumstances.” (Saint Thomas Aquinas, Summa Theologica, II-II, Q. 154, article 12).
“Consequently, circumstances or intentions can never transform an act, intrinsically evil by virtue of its object, into an act ‘subjectively’ good or defensible as a choice.” (Pope John Paul II, Veritatis Splendor, n. 81)
“No circumstance, no purpose, no law whatsoever can ever make licit an act which is intrinsically illicit, since it is contrary to the Law of God which is written in every human heart, knowable by reason itself, and proclaimed by the Church.” (Pope John Paul II, Evangelium Vitae, n. 62)
“But no reason, however grave, may be put forward by which anything intrinsically against nature may become conformable to nature and morally good. Since, therefore, the conjugal act is destined primarily by nature for the begetting of children, those who, in exercising it, deliberately frustrate its natural power and purpose, sin against nature and commit a deed which is shameful and intrinsically vicious.” (Pope Pius XI, Casti Connubii, n. 54)
“No difficulty can arise that justifies the putting aside of the law of God which forbids all acts intrinsically evil. There is no possible circumstance in which husband and wife cannot, strengthened by the grace of God, fulfill faithfully their duties and preserve in wedlock their chastity unspotted.” (Pope Pius XI, Casti Connubii, n. 61)
“For if with the aid of reason and of free will they are to control their natural drives, there can be no doubt at all of the need for self-denial. Only then will the expression of love, essential to married life, conform to right order.” (Pope Paul VI, Humanae Vitae, n. 21)