This poem <The Flea> is all about the persuasion, the secular persuasion that the speaker trying to convince his lover to make love together. In this process of persuasion, he is using the image of a flea. How could ‘Flea’ work, as an excuse of arguing that the two of them should have a love affair, which seemingly have no relations?
Since the flea sucked first his blood, then her blood, so that their blood is mixed together (three lives in one flea sphere), he says that they have already been made as one in the body of the flea.
Introduction to English literature, Ewha institute of American studies, 2013
“The Flea Summary”. Sparknotes, 12 June 2015