Chapter 9 [VIII.]— This Disease of Concupiscence in Marriage is Not to Be a Matter of Will, But of Necessity; What Ought to Be the Will of Believers in the Use of Matrimony; Who is to Be Regarded as Using, and Not Succumbing To, the Evil of Concupiscence; How the Holy Fathers of the Old Testament Formerly Used Wives.
This disease of concupiscence is what the apostle refers to, when, speaking to married believers, he says: “This is the will of God, even your sanctification, that you should abstain from fornication: that every one of you should know how to possess his vessel in sanctification and honour; not in the disease of desire, even as the Gentiles which know not God.” The married believer, therefore, must not only not use another man's vessel, which is what they do who lust after others' wives; but he must know that even his own vessel is not to be possessed in the disease of carnal concupiscence.
And this counsel is not to be understood as if the apostle prohibited conjugal— that is to say, lawful and honourable— cohabitation; but so as that that cohabitation (which would have no adjunct of unwholesome lust, were it not that man's perfect freedom of choice had become by preceding sin so disabled that it has this fatal adjunct) should not be a matter of will, but of necessity, without which, nevertheless, it would be impossible to attain to the fruition of the will itself in the procreation of children.
And this wish is not in the marriages of believers determined by the purpose of having such children born as shall pass through life in this present world, but such as shall be born again in Christ, and remain in Him for evermore. Now if this result should come about, the reward of a full felicity will spring from marriage; but if such result be not realized, there will yet ensue to the married pair the peace of their good will. Whosoever possesses his vessel (that is, his wife) with this intention of heart, certainly does not possess her in the “disease of desire,” as the Gentiles which know not God, but in sanctification and honour, as believers who hope in God.
A man turns to use the evil of concupiscence, and is not overcome by it, when he bridles and restrains its rage, as it works in inordinate and indecorous motions; and never relaxes his hold upon it except when intent on offspring, and then controls and applies it to the carnal generation of children to be spiritually regenerated, not to the subjection of the spirit to the flesh in a sordid servitude. That the holy fathers of olden times after Abraham, and before him, to whom God gave His testimony that “they pleased Him,” thus used their wives, no one who is a Christian ought to doubt, since it was permitted to certain individuals among them to have a plurality of wives, where the reason was for the multiplication of their offspring, not the desire of varying gratification.
Source: On Marriage and Concupiscence (New Advent)