The "marriage hearse" appears in the last line of Blake's poem. The image is deeply ambiguous. First, it conflates two opposites. Marriage is associated with beginnings, procreation, and a kind of socially accepted sexuality. A hearse, of course, is associated with death, trips to the graveyard, and corpses. By bringing these two ideas together, Blake is suggesting that marriage is like a hearse, a journey to the grave.

This "blight" is brought on by the "youthful Harlots curse," a reference to venereal disease. "Youthful harlot" is another phrase, like "marriage hearse," that juxtaposes two opposite connotations. The suggestion is that the innocence and purity associated with "youth" has been perverted by society into prostitution. Marriage becomes a "hearse" because of the diseases spread through the "Harlots" to their customers that are then spread to infants. Marriage, in this sense, leads to disease and tragedy.

Blake was no prude. He believed that sex was an essential part of human spirituality. The main theme of "London" is that a false morality and institutions (the "mind forg'd manacles") have made life a misery. From this point of view, the "harlots curse" could be seen as not an indictment of immorality, but the result of social and religious morality that, some would say, require men to seek out prostitutes to gratify their natural desires.