Claim. Conjugal love is structurally different from parent-child love: it cannot be shared with anyone outside the marriage without being ruined. Absolute exclusivity is therefore not a moralistic add-on but a principle-of-creation.
Elaboration. SMM contrasts the two love-axes: “The horizontal relationship of love between husband and wife is different from the vertical relationship of love between parents and children, in that once it is shared with others, it becomes ruined. This is because, by virtue of the principle of creation, it becomes necessary for husband and wife to form an absolute unity in love. Human beings have the responsibility to live for the sake of their spouses absolutely” (3.3. The importance of ideal marriage, 277-200, 1996.4.16). The aesthetic image is striking: “For God, the sight of a man and woman receiving the Blessing and feeling joy by giving and receiving perfected love is like a flower blossoming on earth. Moreover, everything that occurs through their love is like the fragrance of flowers to God” (ibid, Blessed Family 889). The implication is that conjugal exclusivity, far from being a Christian carry-over, is a built-in principle: parental love expands by sharing; spousal love collapses by sharing.
See also. family, principle-of-creation