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.