Could any realistic narrator be such a fool?

I’ve started (re)reading Ford Madox Ford’s novel The Good Soldier (ca 1915) and am not sure why I remember it as a “great novel.” The writing is fine and clear, a first-person narrative (I don’t think we even know the narrator’s name; does someone call him “John” at one point?) in which we are much more aware than the narrator is about what’s going on. He recounts what he notes in the first sentence is the “saddest story” he’d ever heard – a great way to start a novel, by the way – and slowly puts together the pieces of his unhappy marriage (to a fellow American blue-blood, Florence) and their 11 year friendship w/ a wealthy and self-centered English couple, Capt Ashburnham and his wife, Leonora, whom they meet at a German health spa. We learn, in the first third or so of the novel, that the narrator’s marriage has been a disaster: He married her in haste, hardly knowing her, inexperienced in love and in sex, and on their “honeymoon” sea voyage to Europe she becomes “ill” on the boat and for the next 11 years suffers from heart issues, leading to numerous stays in various European clinics and to a loveless (and sexless) marriage. But guess what? – over all this time Florence has been carrying on an affair w/ Capt. A. – and the narrator never knows or notices anything strange? Come on! Moreover, he’s well aware that the Captain has been involved w/ numerous affairs and scandals, including at least one notorious liaison that led to his leaving his post in India into what seems to be an enforced retirement. In short, the narrator cannot put 2 and 2, or even 1 and 1, together; he entered into a marriage doomed from the start, and he’s either to stupid (which does not seem to be the case, based on his writing/story-telling ability) or afraid and embarrassed to see what’s happening in his life and the lives of those closest to him. I don’t know how to explain this wilful blindness.