Yet thou triumph’st, and say'st that thou <br /> Find’st not thy self, nor me the weaker now;

In line 26, the listener explains how they feel no guilt, and there is no consequence from taking the flea's life as it took both their blood.

’Tis true; then learn how false, fears be:

In line 27, the speaker agrees how there is no consequence and seems to fear something, but, that something is unknown.

Just so much honor, when thou yield’st to me, Will waste, as this flea’s death took life from thee.

In connection to line 27, it seems what seems to be getting lost is the listener's honor. The speaker (?) seems to connect this, comparing the fear of loosing her honor in connection to the flea's death. By saying this, the speaker goes on to push, saying that similar to the flea's death having no consequence, their will be no consequence to them being intimate.

Yet thou triumph’st, and say'st that thou <br /> Find’st not thy self, nor me the weaker now;

In lines 23 and 24, the listener finally speaks, claiming that by killing the flea, she is not weak. By saying this, she essentially says that there was no harm in killing the flea, as it took both their blood.

Cruel and sudden, hast thou since Purpled thy nail, in blood of innocence?

In line 19, the speaker calls the listener "Cruel and sudden" because they killed the flea. In line 20, the speaker paints the picture of the flea's blood painting the purple nail of the listener, showing the violence and lost of a life.