The poem looks back on a moment of sudden desire between two people, and the odd thing is that they laughed at it. The want came so fast and so awkwardly that it seemed almost comic, not romantic. The speaker calls it "poorly formed," as if the feeling showed up half-built.
The images that follow are strange and a little cold. Desire is "a ship turned out of relation / to what satisfied," which suggests something adrift, pointed away from anything that could actually fulfil it. Then comes the final comparison: an appointment "at the grave of a clerk." That picture is dry, bureaucratic, and faintly absurd, and it makes the whole encounter feel dutiful and hollow, as though they were keeping a meeting with something already dead.
The mood is wry and detached, with sadness underneath. The title, "Careless lovers," fits: the speaker seems to see the couple as thoughtless, and perhaps a bit empty, even while remembering them with dark humor.