"Conjugal love, or the friendship of spouses, can persist even after sexual desires have weakened, withered, and disappeared"

Mortimer Adler is stating that the love in between spouses, likewise called conjugal love, can withstand even when libido has faded over time. This shows that real love and affection in between partners exceeds physical tourist attraction and libido. Even after these elements of the relationship have reduced, the emotional and intellectual bond might still be strong in between the two individuals.