Saint Augustine of Hippo, Doctor of the Church, in his moral treatise 'On the Good of Marriage,' writes on the subject of sexual intercourse within marriage:
"…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]
The expression 'that use which is against nature' refers to unnatural sexual acts, such as oral sex, anal sex, or manual sex. Saint Augustine condemns such acts unequivocally. He even states that such unnatural sexual acts are even more damnable (i.e. even more serious mortal sins) when these take place within marriage. For God is even more offended by a sexual mortal sin that takes place within the Sacrament of Marriage, since this offense is not only against nature, but also against a Holy Sacrament. "So then, of all to whom much has been given, much will be required. And of those to whom much has been entrusted, even more will be asked." (Lk 12:48).
Notice that Augustine references the teaching of Sacred Scripture in the words of Saint Paul:
Saint Paul is referring to homosexual sexual acts, between two men or between two women. The "natural use of the body" refers to natural intercourse, between a man and woman. That "use which is against nature" refers to sexual acts between two women or two men. Scripture describes this as "males doing with males what is disgraceful".
But Saint Augustine extends this condemnation of unnatural sexual acts to a man and woman who are married in the Church. He states that it is a grave sin to change "the natural use", meaning natural marital relations, into "that which is against nature," meaning unnatural sexual acts in marriage. These acts are still a mortal sin when done between a man and woman, even if they are married. In fact, Augustine sees these sinful acts are more gravely immoral when done in marriage. For the offense is against nature and against the Sacrament of Marriage.
Therefore, in no way, at no time, can any husband or wife justify the use of unnatural sexual acts in marriage, not for any purpose, not in any circumstance. Such acts are intrinsically evil and always gravely immoral.
Augustine continues on the same topic:
"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]
In this passage, Saint Augustine first compares natural sexual relations within marriage, done out of impure desires ("beyond the necessity of begetting"), to the same natural sexual acts outside of marriage. He teaches that having natural sexual relations within marriage, when done to satisfy a somewhat impure desire, is pardonable, i.e. a venial sin, but that natural sexual relations outside of marriage is damnable, i.e. a mortal sin.
Then Saint Augustine goes on to consider "that which is against nature," i.e. unnatural sexual acts. He condemns such unnatural sexual acts as "execrable" (utterly detestable, abominable, abhorrent). Therefore these acts are among the worst of the sexual mortal sins. He also teaches that unnatural sexual acts within marriage, far from being permitted because they take place within marriage, are even worse, calling them "even more execrable," than the same unnatural sexual acts outside of marriage. Again, this is because the sin is not only against nature, but against a Holy Sacrament instituted by Christ himself for the sake of our salvation.
Therefore, unnatural sexual acts do not become permissible when these take place within marriage. Instead, unnatural sexual acts are made even more sinful when these take place within marriage because they offend against both nature and a Sacrament.
More on marital chastity in The Catholic Marriage Bed: A book of Roman Catholic moral theology.
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