[This and the previous article are omitted in the Leonine edition.]
Objection 1. It would seem that a menstruous wife may not pay the marriage debt to her husband at his asking. For it is written (Leviticus 20:18) that if any man approach to a menstruous woman both shall be put to death. Therefore it would seem that both he who asks and she who grants are guilty of mortal sin.
Objection 2. Further, "Not only they that do them but they also that consent to them are worthy of death" (Romans 1:32). Now he who knowingly asks for the debt from a menstruous woman sins mortally. Therefore she also sins mortally by consenting to pay the debt.
Objection 3. Further, a madman must not be given back his sword lest he kill himself or another. Therefore in like manner neither should a wife give her body to her husband during her menses, lest he be guilty of spiritual murder.
On the contrary, "The wife hath not power of her own body, but the husband" (1 Corinthians 7:4). Therefore at his asking his wife must pay the debt even during her menses.
Further, the menstruous wife should not be an occasion of sin to her husband. But she would give her husband an occasion of sin, if she paid him not the debt at his asking; since he might commit fornication. Therefore, etc.
I answer that, In this regard some have asserted that a menstruous woman may not pay the debt even as she may not ask for it. For just as she would not be bound to pay it if she had some personal ailment so as to make it dangerous for herself, so is she not bound to pay for fear of danger to the offspring. But this opinion would seem to derogate from marriage, by which the husband is given entire power of his wife's body with regard to the marriage act. Nor is there any parallel between bodily affliction of the offspring and the danger to her own body: since, if the wife be ailing, it is quite certain that she would be endangered by the carnal act, whereas this is by no means so certain with regard to the offspring which perhaps would not be forthcoming.
Wherefore others say that a menstruous woman is never allowed to ask for the debt; and that if her husband ask, he does so either knowingly or in ignorance. If knowingly, she ought to dissuade him by her prayers and admonitions; yet not so insistently as possibly to afford him an occasion of falling into other, and those sinful, practices, if he be deemed that way inclined. If however, he ask in ignorance, the wife may put forward some motive, or allege sickness as a reason for not paying the debt, unless there be fear of danger to her husband. If, however, the husband ultimately persists in his request, she must yield to his demand. But it would not be safe for her to make known ["Indicare," as in the commentary on the Sentences; the Leonine edition reads "judicare."] her disaffection, lest this make her husband entertain a repulsion towards her, unless his prudence may be taken for granted.
Reply to Objection 1. This refers to the case when both willingly consent, but not when the woman pays the debt by force as it were.
Reply to Objection 2. Since there is no consent without the concurrence of the will, the woman is not deemed to consent in her husband's sin unless she pay the debt willingly. For when she is unwilling she is passive rather than consenting.
Reply to Objection 3. A madman should be given back his sword if a greater danger were feared from its not being returned to him: and thus it is in the case in point.
Source: Summa Theologica (New Advent)