First, I’d like to point out that actively spacing births IS birth control. It is synonymous with “birth regulation”.
Second, the Church does not condemn birth control/regulation…it condemns contraception. In fact, the Church refers to birth control (usinng natural methods) as “responsible parenthood” (CCC 2368 and 2399).
Third, the Church does NOT say the “just reason” must be a “serious medical condition”. Your Pastor is not in conformity with the Church if he says that one needs a “serious medical condition” to avoid mortal sin. See CCC 2370, Casti connubii 59, or Pope Pius’ “Allocution to Midwives” which states very cleary, “…
Serious motives, such as those which not rarely arise from
medical, eugenic, economic and social so-called “indications,” may exempt husband and wife from the obligatory, positive debt for a long period or even for the entire period of matrimonial life. From this it follows that the observance of the natural sterile periods may be lawful, from the moral viewpoint: and it is lawful in the conditions mentioned…”
Only the couple knows God’s plans for that couple’s family. Your Pastor is mistaken.
The OP did state the couple has no serious reason to avoid, yet they purposefully avoid. For that reason, in this hypothetical scenario, I would agree here that the couple is not in conformity with the purpose of marriage. However, in a real life situation, only that couple knows whether they have a just reason to avoid after 2 children. Maybe it’s the argue’er in me…but can any of us say that this couple is closed to the gift of children after having 2 children? What is the minimum number of children that a couple must have in order to show they are open to the gift of children? (No such number exists.)
The OP clarifies with this:
…
they just don’t want the extra headache and physical running around which is quite demanding. This may sound selfish but it gets tiring one day after the next after the next after the next.
Who is to say that this is not a just reason to avoid? The Church makes no such claim. If this is a just reason to that couple…then it is a just reason. If they are using NFP to avoid…they have a just reason…otherwise, they’d not be avoiding during the fertile period…they’d be using contraception instead.