as the symbol for all that is wrong with England, Blake is able to condemn institutions and emotions that are sacred to most people: love and marriage.

Henrik Ibsen's play Ghosts (1882) is about how a young man's life is ruined by the fact that his father passed on the disease of syphilis to him at conception some twenty-six years earlier.  

The enotes Study Guide to Blake's "London" says the following concerning the lines you ask about:

The final stanza of the poem is set in darkness—Blake is listening in the midnight streets to the cries of young prostitutes as they curse the men who victimize them, the wives who are equally victims, and the religion that forces people to think that they must marry and stay married no matter what. “London” ends on a pessimistic note in which Blake reviles the one sacrament that should offer hope to present and future generations: marriage. Instead of being predicated on love and mutual respect, Blake sees it as something that enslaves the body and soul in much the same way that stanzas 2 and 3 point out that English laws victimize the less fortunate.

This final stanza reveals what sex, marriage and birth lead to in London.  Blake offers in this work, as he does in numerous poems, a perspective that is opposite the usual.  His collections, Songs of Innocence and Songs of Experience demonstrate how multiple perspectives exist for similar situations and circumstances.  "London," which belongs to Experience, demonstrates a perspective that understands the dark side of London, a city that is not only the capitol of England, but the capitol of the British Empire.

This poem is meant to show how bad life is in London.  The lines that you mention help to paint this picture.

Harlots are prostitutes -- that is important to understanding these lines.

Blake is saying that, as he walks through London, he hears and sees many bad things.  The last of these are described in the words you mention.

There, he is describing the cursing of a young prostitute, presumably as she gives birth to a baby.  Her baby's innocence is lost because of her cursing.  This shows how even at the beginning of life, things are already bad for the poor of London.

William Blake published "London" in 1794 in his Songs of Experience. The French Revolution had taken place just five years earlier, and this caused lawmakers in England to pass new laws that would restrict individual liberties in an effort to avoid something similar happening there. Instead of a place of relative freedom, creativity, and beauty, Blake seems to feel that England—and London in particular—had turned into a place where people are so oppressed by so many forces that they cannot be anything but miserable.

In the poem, seemingly everything is "charter'd"—the streets and the Thames. So, it would seem, are Londoners' lives, so there is no possibility of freedom or hope or change. Hemmed in by barriers of all kinds, people are marked by their woe and reduced to weakness. They each figuratively wear the "mind-forg'd manacles" that symbolize their captivity and their powerlessness against those forces that control them.

The church is "blackning," implying how it has descended into a state of moral decay, perhaps reflected, in part, by the practice of allowing the children in its care to be exploited as chimney sweeps, who themselves are literally blackened by the soot of their trade. Then, the "hapless" and unfortunate soldiers have no say in what they must do, implying that they must carry out the unjust or violent demands of their government, symbolized by the "Palace."

Finally, not even new marriages or innocent babies can escape the corruption and hopelessness of London during this era, as the prostitutes' curses "blast" the infants and "blight" the "marriage hearse," a word very much associated with death. During this time in London, the city hosted a great number of prostitutes, and venereal diseases were often spread from them to their customers and even their customers' unborn children. This scene further emphasizes people's lack of personal freedoms, the moral corruption of the church and state, the feeling of being trapped in a terrible situation, the suffering of all, and the sense that death is the only escape.

Blake's "London" present a bleak portrait of life in the capital city. The speaker notes marks of "weakness" and unhappiness in "every" face he sees as he walks the city's streets. His London is filled with the "cry" of suffering. The word cry, encompassing both the idea of tears and the utterance of lament, is repeated three times in these verses.

The speaker blames human institutions for the suffering he perceives all around him. The church and the aristocracy are to blame for the misery of London life. This social state is manmade, not innate or natural. Because society does not have to be this way, the poem implies, it could and should be changed.

The speaker especially points to the "cry" of the chimney sweepers, young boys from impoverished backgrounds who were kept underfed so they could slide down and clean narrow chimneys. In this case, the word cry is a pun referring both to the way the sweepers would cry or call out their availability to anyone who might hire them and to their crying because of the pain and sadness of their lives. The speaker also singles out the "harlots," or prostitutes, who come out at night and spread venereal diseases and blight through their work.

Overall, the poem presents London, a symbol of civilization and empire, as a place of misery.

The imagery in "London" is largely visual or auditory. This means that it conveys sensory information that one can either imagine seeing or hearing, respectively.

In the second stanza, the speaker mentions "every cry of every Man" and "every Infants cry of fear," prompting the reader to imagine the sound of a cacophony of cries—of huge crowds of people, made up of the young and old, in which each person is crying aloud to him or herself in his or her pain and suffering.

Further, the speaker references the "mind-forg'd manacles" figuratively worn by each inhabitant of the city. The word manacles is so specific in itself that the reader immediately conjures the visual picture of the iron restraints used to shackle and control enslaved people or prisoners.

In the third stanza, the visual image of the "blackning Church" describes the way in which soot might build up on church walls. Figuratively, this conveys the corruption of the church during this era, the neglect of the church's principles by the people, or perhaps both. Next, the auditory image of the "Soldiers sigh" is immediately followed by the frightening visual image of "blood [running] down Palace walls."

In the fourth and final stanza, the speaker describes the sound of the "youthful Harlots curse," creating yet another auditory image. Finally, the "Marriage hearse" conjures a visual image of a black mourning vehicle meant to carry a dead body but that, in this case, doubles as a carriage to carry a newly-married couple—a foreboding and ominous image, to say the least.

William Blake’s “London” is a poem about contemporary life at the time as it is characterized by certain forces that affect social, intellectual and spiritual life. The city, London, is used to represent these forces, many of which are institutional. Urbanization, to use a modern term, is part of what Blake is criticizing in this poem about the dehumanizing influence of a prevailing commercial and religious ethos that seeks to shape individuals into conformity.

This is a poem about ideology. Ideology, in “London,” is evoked in the phrase, “mind forg’d manacles.” This phrase speaks to the notion that the institutions governing behavior and defining social norms come to exist in the minds of the citizens of the city. People’s thinking is controlled, to a significant extent, for Blake, by the dictates of the Church and the State.

In every cry of every Man,
In every Infants cry of fear,
In every voice: in every ban,
The mind-forg'd manacles I hear

The suffering that results from the imposition of the collectively defined ideology relates closely to commerce and economically driven constraints.

Forced to sell her body to make money, ostensibly to simply survive, “the youthful Harlots curse/Blasts the new-born Infants tear/And blights with plagues the Marriage hearse.” Thus the life of the prostitute is rendered miserable by the need for money while sexually transmitted disease in turn plagues the new marriage of those who are lucky enough to be in the position to marry.

There is a cycle at work, suggested in the last stanza. This cycle knits together the misery of the innocent (the infant, the new marriage) along with the “experienced,” as it were. The Church in the poem does not bring light but is described as “blackening” and so can be seen to take part also in this fusion or inversion of innocence and experience.

The professions mentioned in the poem, from soldier to chimney sweep, are depicted negatively. Manacles of economic origin thus parallel those of the mind. The idea that animates the city – an idea relating to commerce and economic vigor – is ultimately connected to an idea of spiritual death.

So, again, the themes of the poem relate to the ways that, for Blake, the doctrines of commerce that define London can function to constrain or imprison individuals into roles that may be rigid, unpleasant, and/or harmful to the life of the spirit. In some ways the citizens are enslaved to a system of commerce wherein there is no chance for joy or innocence.