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!
It is not in line with Catholic teaching to avoid conception without a serious reason. Have you read the Catechism? Have you read "Human Vitae"? It is not "Catholic teaching" to use natural methods to avoid conception universally.
If they wanted to be "in line with Catholic teaching" straight up, they would view the primary end of their marriage as the procreation and education of children, and would therefore have as many children as naturally resulted during the marriage. The use of NFP to avoid pregnancy is AN EXCEPTION to the generalization that children are an unmerited gift from God, for which we should long, which we should accept without reservation.