Objection 1. It would seem that husband and wife are not mutually bound, under the obligation of a precept, to the payment of the marriage debt. For no one is forbidden to receive the Eucharist on account of fulfilling a precept. Yet he who has had intercourse with his wife cannot partake of the flesh of the Lamb according to Jerome [Serm. de Esu Agni viii] quoted in the text (Sent. iv, D, 32). Therefore the payment of the debt does not come under the obligation of a precept.
Objection 2. Further, it is lawful to everyone to abstain from what is hurtful to his person. But it is sometimes harmful to a person to pay the debt when asked, whether on account of sickness, or because they have already paid it. Therefore it would seem allowable to refuse the one who asks.
Objection 3. Further, it is a sin to render oneself unfit to fulfill an obligation of precept. If, therefore, the payment of the debt comes under the obligation of a precept, it would seem sinful to render oneself unfit for paying the debt, by fasting or otherwise weakening the body: but apparently this is untrue.
Objection 4. Further, according to the Philosopher (Ethic. viii, 12), marriage is directed to the begetting and rearing of children, as well as to the community of life. Now leprosy is opposed to both these ends of marriage, for since it is a contagious disease, the wife is not bound to cohabit with a leprous husband; and besides this disease is often transmitted to the offspring. Therefore it would seem that a wife is not bound to pay the debt to a leprous husband.
On the contrary, As the slave is in the power of his master, so is one spouse in the power of the other (1 Corinthians 7:4). But a slave is bound by an obligation of precept to pay his master the debt of his service according to Romans 13:7, "Render... to all men their dues, tribute to whom tribute is due," etc. Therefore husband and wife are mutually bound to the payment of the marriage debt.
Further, marriage is directed to the avoiding of fornication (1 Corinthians 7:2). But this could not be the effect of marriage, if the one were not bound to pay the debt to the other when the latter is troubled with concupiscence. Therefore the payment of the debt is an obligation of precept.
I answer that, Marriage was instituted especially as fulfilling an office of nature. Wherefore in its act the movement of nature must be observed according to which the nutritive power administers to the generative power that alone which is in excess of what is required for the preservation of the individual: for the natural order requires that a thing should be first perfected in itself, and that afterwards it should communicate of its perfection to others: and this is also the order of charity which perfects nature. And therefore, since the wife has power over her husband only in relation to the generative power and not in relation to things directed to the preservation of the individual, the husband is bound to pay the debt to his wife, in matters pertaining to the begetting of children, with due regard however to his own welfare.
Reply to Objection 1. It is possible through fulfilling a precept to render oneself unfit for the exercise of a sacred duty: thus a judge becomes irregular by sentencing a man to death. In like manner he who pays the marriage debt, in fulfillment of the precept, becomes unfit for the exercise of divine offices, not because the act in question is sinful, but on account of its carnal nature. And so, according to the Master (Sent. iv, D, 32), Jerome is speaking only of the ministers of the Church, and not of others who should be left to use their own discretion, because without sin they may either abstain out of reverence or receive Christ's body out of devotion.
Reply to Objection 2. The wife has no power over her husband's body, except as is consistent with the welfare of his person, as stated above. Wherefore if she go beyond this in her demands, it is not a request for the debt, but an unjust exaction; and for this reason the husband is not bound to satisfy her.
Reply to Objection 3. If the husband be rendered incapable of paying the debt through a cause consequent upon marriage, for instance through having already paid the debt and being unable to pay it, the wife has no right to ask again, and in doing so she behaves as a harlot rather than as a wife. But if he be rendered incapable through some other cause, then if this be a lawful cause, he is not bound, and she cannot ask, but if it be an unlawful cause, then he sins, and his wife's sin, should she fall into fornication on this account, is somewhat imputable to him. Hence he should endeavor to do his best that his wife may remain continent.
Reply to Objection 4. Leprosy voids a betrothal but not a marriage. Wherefore a wife is bound to pay the debt even to a leprous husband. But she is not bound to cohabit with him, because she is not so liable to infection from marital intercourse as from continual cohabitation. And though the child begotten of them be diseased, it is better to be thus than not at all.
Source: Summa Theologica (New Advent)