It could be that your "Couple A" has a serious medical reason for avoiding pregnancy. Or perhaps they have a disastrous marriage, but decide not to divorce due to their belief in the sanctity of their vows, in spite of the outcome. It is possible that their motives for avoiding pregnancy are within what the Church would consider "serious reasons." (Have you read "Humanae Vitae"? That would help a lot!)
Your Couple B is practicing sin, irrespective of the outcome. The seriousness of their sin depends on their level of knowledge and the nature of the relationship; as you know, mortal sin requires "Grave matter," which this is, but also "Full knowledge," and "Full consent of the will." From a personal standpoint (pardon my psychological baggage) I would not let a man use me as a toilet year after year, without being open to having children. He could have affairs, he could file for divorce, he could climb Mount Everest, but I simply wouldn't stand for it.
It could be that your "Couple A" has a serious medical reason for avoiding pregnancy. Or perhaps they have a disastrous marriage, but decide not to divorce due to their belief in the sanctity of their vows, in spite of the outcome. It is possible that their motives for avoiding pregnancy are within what the Church would consider "serious reasons." (Have you read "Humanae Vitae"? That would help a lot!)"
What if that couple just wanted to be in line with the Catholic teaching....Nothing more!
If there are forseen medical conditions which would prevent the bearing of children should they be conceived, no marriage may be permitted to take place.
Or perhaps they have a disastrous marriage, but decide not to divorce due to their belief in the sanctity of their vows, in spite of the outcome.
It is difficult to understand how this could actually be the case.