The speaker attends their father's wedding, but not as an ordinary guest. They arrive as a "long white winter, a doubt, a fruitless precaution", a set of images that turns the speaker into a kind of chill moving through the celebration. Winter suggests coldness and distance, while "doubt" hints at suspicion about whether this marriage will last or be good. A "fruitless precaution" is something done to guard against a danger that doesn't matter in the end. Judging by the wording, the speaker may have felt wary or protective, then realized it changed nothing.
The final line, "Life did not stop," is short and flat. It could carry relief, disappointment, or a bit of both. The speaker's gloom, however strong, had no power over the event. The mood is quiet, wry, and a little lonely: a private reluctance set against a world that simply carries on. The title, "Conceivable," might nod to what the speaker could imagine going wrong, though the poem never says so outright.