When a man is faithful to one woman, he looks on other women in quite another way, a way unknown to the world of Eros; other women turn into persons instead of being reflections or means. This ‘spiritual exercise’ develops new powers of judgment, self-possession, and respect.* The opposite in this of an erotic man, a steadfast man no longer strives to see a woman as merely an attractive or desirable body... he feels, as soon as tempted, he has been desiring only an illusory or fleeting aspect of what is actually a complete life. Thus temptation recedes disconnected instead of making itself into an obsession; and fidelity is made secure by the clear-sightedness it induces.
(*‘Respect’, as I use the word here, means that we recognize in a being the fullness of a person. A person, according to Kant’s famous definition, is what cannot be used by man as an instrument or thing.)
Denis de Rougemont, Love in the Western World