Objection 1. It would seem that it is a mortal sin to ask for the debt at a holy time. For Gregory says (Dial. i) that the devil took possession of a woman who had intercourse with her husband at night and came in the morning to the procession. But this would not have happened had she not sinned mortally. Therefore, etc.
Objection 2. Further, whoever disobeys a Divine command commits a mortal sin. Now the Lord commanded (Exodus 19:15): "Come not near your wives," when namely they were about to receive the Law. Much more therefore do husbands sin mortally if they have intercourse with their wives at a time when they should be intent on the sacred observances of the New Law.
On the contrary, No circumstance aggravates infinitely. But undue time is a circumstance. Therefore it does not aggravate a sin infinitely, so as to make mortal what was otherwise venial.
I answer that, To ask for the debt on a feast day is not a circumstance drawing a sin into another species; wherefore it cannot aggravate infinitely. Consequently a wife or husband does not sin mortally by asking for the debt on a feast day. It is however a more grievous sin to ask for the sake of mere pleasure, than through fear of the weakness of the flesh.
Reply to Objection 1. This woman was punished not because she paid the debt, but because afterwards she rashly intruded into the divine service against her conscience.
Reply to Objection 2. The authority quoted shows not that it is a mortal sin but that it is unbecoming. For under the Old Law which was given to a carnal people many things were required under an obligation of precept, for the sake of bodily cleanness, which are not required in the New Law which is the law of the spirit.
Source: Summa Theologica (New Advent)