First, St. Thomas quotes Peter Lombard: Conjugal relations "are not to be changed to a use contrary to nature" [The Sentences, Book IV]. Then, St. Thomas explains what Lombard meant and why he is correct:
"Marital relations are contrary to nature when either the right receptacle or the proper position required by nature is avoided. In the first case, it is always a mortal sin because no offspring can result, so that the purpose of nature is completely frustrated (Unde totaliter intentio naturae frustratur). But in the second case it is not always a mortal sin, as some say, though it can be the sign of a passion which is mortal; at times the latter can occur without sin, as when one’s bodily condition does not permit any other method. In general, this practice is more serious the more it departs from the natural way." [In Libros Sententiarum, IV, 31, 2, 3]
All unnatural sexual acts are intrinsically evil and always gravely immoral, even in marriage. Spouses may not use unnatural acts as foreplay, or for any other reason. The use of the "right receptacle" is a modest way of referring to natural relations (genital-to-genital). Unnatural sexual acts include oral, anal, manual, or any other act or position that is not natural intercourse, because the acts are not ordered toward procreation and the natural purpose of sexual relations is entirely thwarted. Such acts are "contrary to nature", as both Peter Lombard and Saint Thomas assert.
Saint Thomas allows that sexual positions, other than the "proper position" [missionary position], can be morally permissible, only if it is a natural position (genital-to-genital), i.e. a position in itself capable of procreation. If various natural positions are used due to "a passion which is mortal" (i.e. out of lust), then the act is a grave sin. But a natural position can be used without sin, as when "one’s bodily condition does not permit any other method." Even so, any unnatural sexual position or unnatural sexual act is intrinsically evil and always gravely immoral, regardless of this circumstance of bodily condition because it is "always a mortal sin" when the sexual act is inherently closed to procreation "so that the purpose of nature is completely frustrated."
Notice, in the next quote below, that St. Thomas held sexual sins within marriage to be worse than adultery, because the act occurs within the good of marriage. He did not teach that all sexual acts within marriage are moral, nor did he teach that all sexual acts between a husband and wife are moral.
"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]
The phrasing "if he use her indecently" refers to unnatural sexual acts within marriage. This is clear because the good of marriage emphasized by St. Thomas is the procreation of children (Summa Theologica, II-II, Q. 154, article 2). St. Thomas could not be referring to natural marital relations when he says "if he use her indecently" because even natural marital relations done with some disorder of desire still retains the unitive and procreative meanings. But unnatural sexual acts lack both meanings, and so they are contrary to the good of marriage. The use of unnatural sexual acts within marriage is therefore worse than adultery.
St. Thomas again condemns this same type of act later in the same question.
"Lastly comes the sin of not observing the right manner of copulation, which is more grievous if the abuse regards the 'vas' than if it affects the manner of copulation in respect of other circumstances." [Saint Thomas Aquinas, Summa Theologica, II-II, Q. 154, article 12]
First, the word 'vas' is Latin for vessel, referring to the use of other bodily orifices for sexual acts. If a husband treats his wife lustfully during natural marital relations, he sins. But he commits a more grievous offense, which is called by St. Thomas an abuse, if he sins by committing unnatural sexual acts (i.e. using an unnatural part of the body as a 'vessel' for sexual intercourse). Here St. Thomas explicitly (but in discrete language) condemns the sin of unnatural sexual acts within marriage.
Second, it is clear (in the quote from article 8 above) that St. Thomas taught that a married couple is not justified in committing any sexual acts whatsoever within marriage. Otherwise, he would not have taught that a man who is too ardent a lover of his wife commits a sin that is like adultery and yet worse than adultery. Therefore, those who claim that there are no sins for a husband and wife having sexual relations with each other are in error.
Third, neither does St. Thomas even consider the absurd argument that acts which are intrinsically evil and gravely immoral by themselves could become good and moral when combined in some way with natural marital relations open to life. If this were the case, then St. Thomas could not have compared a man who is too ardent a lover of his wife to an adulterer. For if he took the position of these modern-day moral theologians, then he would have to say that a husband's ardent love would be entirely justified, as long as "the semen are not misdirected." Notice that Saint Thomas takes no such position; he does not sum up the marital act as merely the proper direction of semen. And his teaching is fundamentally contrary to the set of errors promoted by these theologians.
More on marital chastity in The Catholic Marriage Bed: A book of Roman Catholic moral theology.
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