Jewish law says that if a couple has no children after ten years of marriage, and the man is not allowed to take a second wife, they must get a divorce so the man can remarry and try again to fulfill the mitzvah of procreating. [Yevamot 64a; Rambam, Ishut 15:8]
But the law is not enforced, even when the couple is very observant. The last Lubavitcher Rebbe is a case in point. The Rema says in EH 1:3: "Nowadays, the custom is that one does not force in regards to this." But how can he make optional, with no explanation, a clear Talmudic injunction codified in stone by the Rambam?
Is there a discussion of circumstances under which does the injunction not apply?