[XIV.]— A Certain Degree of Intemperance is to Be Tolerated in the Case of Married Persons; The Use of Matrimony for the Mere Pleasure of Lust is Not Without Sin, But Because of the Nuptial Relation the Sin is Venial
But in the married, as these things are desirable and praiseworthy, so the others are to be tolerated, that no lapse occur into damnable sins; that is, into fornications and adulteries. To escape this evil, even such embraces of husband and wife as have not procreation for their object, but serve an overbearing concupiscence, are permitted, so far as to be within range of forgiveness, though not prescribed by way of commandment: and the married pair are enjoined not to defraud one the other, lest Satan should tempt them by reason of their incontinence. For thus says the Scripture: “Let the husband render unto the wife her due: and likewise also the wife unto the husband.
The wife has not power of her own body, but the husband: and likewise also the husband has not power of his own body, but the wife. Defraud not one the other; except it be with consent for a time, that you may have leisure for prayer; and then come together again, that Satan tempt you not for your incontinency. But I speak this by permission, and not of commandment.” Now in a case where permission must be given, it cannot by any means be contended that there is not some amount of sin.
Since, however, the cohabitation for the purpose of procreating children, which must be admitted to be the proper end of marriage, is not sinful, what is it which the apostle allows to be permissible, but that married persons, when they have not the gift of continence, may require one from the other the due of the flesh— and that not from a wish for procreation, but for the pleasure of concupiscence? This gratification incurs not the imputation of guilt on account of marriage, but receives permission on account of marriage.
This, therefore, must be reckoned among the praises of matrimony; that, on its own account, it makes pardonable that which does not essentially appertain to itself. For the nuptial embrace, which subserves the demands of concupiscence, is so effected as not to impede the child-bearing, which is the end and aim of marriage.
Source: On Marriage and Concupiscence (New Advent)