Of course a couple and only the couple should make the decision. I think Memaws point is that a couple should do this being informed by the Church and what She teaches. I certainly have no problem suggesting that any couple should educate themselves and their souls in this subject and that the Church and Her theology should be the driving force behind informing their consciences. The common answer of “just pray about it” or “just you and your spouse decide.” is just not sufficient. Personally, I think this is all fueled by a “keep God and the Church out of the bedroom” mentality. I think the Church should not only be involved but should be even more involved.
NFP and “grave or serious or just” reasons are left up to the couple. I cannot think of another moral subject where the Church guides us less than this very one. Perhaps because it is a newer subject and the Church acts upon Her own time. But I think in the future we will be given more guidance on this issue. For now we do have the Church to guide and there is some biblical and Church tradition to inform our selves about and to align ourselves with God’s will properly.
I think it is quite wise to advise any couple in any situation to seek out a priest or other Church source to help them decide. This is not just a study and decide in one weekend for all time subject. It should be prayed about, discerned and revisited regularly and in front of the Blessed Sacrament as much as possible!
And that is my advice for a previous poster who tried to get me to pin down medical issues that I deemed worthy to avoid. Each situation is different. If we could construct hypothetical couples we could debate what each one should do. But in my opinion too many people decide that since it is up to each couple and each situation is different that it gives them some kind of blank check with their own morality. It does not.
It also matters how the couple avoids. And in this original situation I think it is a sign of if they are in tune with God’s will for their family is if they are following God’s will in family planning.
I’m assuming they are not. I could be wrong but the OP balked at that subject all together, and not many NFP users make blanket statements like “never have any biological children.” If they are using NFP, they are more confident in their practice of it that any couple I have ever met to make sweeping statements like that!