From a quotation in that article we read “Certainly it is a duty of married couples—
who, for that matter, should seek appropriate counsel—to deliberate deeply and in a spirit of faith about the size of their family, and to decide the concrete mode of realizing it, with respect for the moral criteria of conjugal life”
Morality and canon law are not the same, so something can be a duty without being a canonical obligation. Further, the Church doesn’t typically define things on such a practical level.
For example, try to find me a definition from ecclesiastical authority on what exactly is “meat” for Friday abstinence. You’ll find it in approved moral theologians’ work, but I wouldn’t expect to find a Church definition.
Catholic Encyclopedia - Issued by the Catholic Church
newadvent.org/cathen/01067a.htm
Meat diet comprises the flesh, blood, or marrow of such animals and birds as constitute flesh meat according to the appreciation of intelligent and law-abiding Christians. For this reason the use of fish, vegetables, mollusks, crabs, turtles, frogs, and such-like cold-blooded creatures is not at variance with the law of abstinence.
The default is to have children and one should only space pregnancies if there are just reasons for doing so which are proportionate to the serious duty of marriage. So yes, it would be an accurate summary to say “you’re fertile and you know it, so get to work - unless you have a legitimate reason not to.” A bad reason is not made good by a couple’s fiat, so they would do well to seek advice from orthodox and traditional moral theologians/priests who are trained in these matters.
I am only pointing out that the fact that a couple decides a certain thing constitutes a just cause does not make it in fact a just cause, nor does it mean that their decision is infallible and irreproachable. Therefore it is prudent to seek advice if the matter is uncertain. There are always dangers of deceiving ourselves (or even just honestly messing up) in these matters, but less so if we have the advice of wise people, which is why the Bible tells us that “the way of a fool is right in his own eyes: but he that is wise hearkeneth unto counsels” (Proverbs 12:15).