The poet asks his mistress to notice only this flea, to forget everything else as he delivers his argument. The flea has bitten them both, and their bloods mix within its body. The attention paid to the qualities of blood may be noted here and later in the poem (when the woman suddenly gains a stature of royalty [purpling her nail]). This mixing of bloods is somewhat of an insult to the lady, if she is of royal blood and he is not. The description of the swelling of the insect with "one blood made of two" is suggestive of surrogate pregnancy, a perversion of motherhood. Such an allusion is definitely not a pleasant nor natural one, and it would be natural for the lady to kill the flea out of disgust after hearing these lines. The word "suck" in this context would be equivalent to the experience of passion or lust, which leads to the loss of innocence. The man admits that the flea sucked him first, so he has lost his innocence, but he still finds himself honorable, so here he bases his own point of view.
The flea becomes ultimately a symbol of the world in which the lovers' desires are realized, "this our marriage bed and marriage temple is." Marriage and consummation is a past issue, since within the flea their blood is already mingled and the "child" of their union grows. The flea is now the realm of marriage, all-encompassing the lovers and excluding any parents or patriarchal sanction. The walls of this realm are jet black, indicating that something sinister or evil is to occur here. This could be a reference to the illicit marriage, or the forbidden mixing of royal and common blood, or perhaps only the impiety of the poet's comparison that loss of innocence should be so trivial as the life of a flea. The lady's significance is reduced to that of a black widow spider at this point, where the poet says she is apt to kill him after this consummation of a non-existent marriage. With this metaphor of the spider, who is also jet in color, the object of the man's love is reduced to the position of the flea. If the flea is pregnant with their blood-child, then she (the lady) may as well be pregnant too. Now that this tie has been established between the blood of the woman and the flea, if the woman were to kill the flea, it would be a form of suicide. So to kill this flea, the woman would have to commit murder (of the symbolic marriage realm and the child within), suicide (killing of her own blood), and sacrilege (which suicide is). Apparently the woman kills the flea anyway, since the death of the flea and her own corruption is addressed next.
By killing the flea whom the poet has given such strange attributes, the woman squashes the symbolic world the man has constructed and brought them both back to reality. By murdering the "innocent" flea, the lady has "purpled her nail," a color assigned to the clothing of royalty. She now becomes a monarch (a pun; she is a autonomous ruler and an insect, the butterfly), gaining her position through the death of the flea. She has committed the sins that destroy the union of their blood, so she triumphs. She says that neither of them are any worse for the loss of blood caused by the pest, which the poet confirms to be the truth. This comment suggests that the lady has just admitted her loss of innocence by implying that the flea really didn't do anything to deserve death. The poet finalizes his argument for his cause by granting that the death of the flea is really of no consequence, as are her fears for her honor. Her honor will not waste when she gives in to him.
The hopeful suitor that addresses his honorable lady in "The Flea" argues ingeniously throughout the verses, shifting the limits of a tiny insect to entire world encompassing the couple. He recognizes sexual relationships out of marriage as a sin, and as a shameful act even when legitimate. Thus the act could only be committed symbolically, within the body of a flea. If such a union of the suitor and his lady can be realized in the flea, then let the flea become the entire world, so that their love can be a reality. As the poet relates his vision of their love in a clandestine world, the lady denies him by smashing the flea. Her reasons may be that she is a noble and he is not, so the suggestion that they mix blood is highly insulting, or that the entire subject he is discussing is not modest enough for a maiden. Upon her smashing of his poetic world of marriage and love, the man assures her that what she has done is of no consequence. He compares her fear for her honor to the importance of the now dead flea, which is nothing.