What is the fundamental wisdom behind permitting the first glance, and how does prolonging and scrutinizing it, under the pretext that it is the first glance, contradict the command to lower the gaze? What are the harms of this on married life?

Intentionally looking at a non-mahram woman is absolutely forbidden. As for a sudden glance, it is excusable on condition that one averts their gaze immediately and does not follow it with another glance. Otherwise, one falls into the forbidden, because the Prophet, peace and blessings be upon him, said: "Do not follow a glance with another glance, for the first is yours, but the last is not." And letting one's gaze roam inherits blindness of the heart and weakness of insight.