The speaker opens bluntly: an old man's love is worthless, like a shell nobody was looking for in the sand. The poem then explains why. An old man's desire for a young woman, it suggests, comes from the infant still inside him, the part that wants a breast for comfort. She, meanwhile, shrinks from his closeness because it reminds her of death and bodily decay. Yet the speaker adds a grim twist: her youth won't last, and her breast will one day "droop to suckle" death.
The mood is bleak, unsentimental, and a little bitter. The speaker seems to see no real tenderness here, only fear. The line about consuming each other rather than consummating suggests that people use one another out of panic, clinging to the next breath. The ending strips everything down to "dust, dying suns, and space," leaving a cold, humbling sense that our small desires matter very little.