Objection 1. It would seem that neither are they bound to pay the debt at a festal time. For those who commit a sin as well as those who consent thereto are equally punished (Romans 1:32). But the one who pays the debt consents with the one that asks, who sins. Therefore he sins also.
Objection 2. Further, it is an affirmative precept that binds us to pray, and therefore we are bound to do so at a fixed time. Therefore one ought not to pay the debt at a time when one is bound to pray, as neither ought one at a time when one is bound to fulfill a special duty towards a temporal master.
On the contrary, It is written (1 Corinthians 7:5): "Defraud not one another, except by consent, for a time," etc. Therefore when one spouse asks the other must pay.
I answer that, Since the wife has power of her husband's body, and "vice versa," with regard to the act of procreation, the one is bound to pay the debt to the other, at any season or hour, with due regard to the decorum required in such matters, for this must not be done at once openly.
Reply to Objection 1. As far as he is concerned he does not consent, but grants unwillingly and with grief that which is exacted of him; and consequently he does not sin. For it is ordained by God, on account of the weakness of the flesh, that the debt must always be paid to the one who asks lest he be afforded an occasion of sin.
Reply to Objection 2. No hour is fixed for praying, but that compensation can be made at some other hour; wherefore the argument is not cogent.
Source: Summa Theologica (New Advent)