On what basis does the poet John Donne say that he himself and the girl become one in the flea? [British Poetry]

In John Donne's poem "The Flea," the speaker uses the metaphor of a flea sucking the blood of both himself and the girl he is addressing to argue that their physical union in the flea's act of biting them is a kind of spiritual and physical merging. He is saying that their lives are now intertwined in the flea, and that it would be wrong for them to deny their mutual attraction and not consummate their love. He says that since the flea has already united them in this small way, it would be a greater sin to separate them. He is trying to persuade the girl to give in to their desire and become sexually intimate by emphasizing that their union in the flea is a symbol of their spiritual and physical unity.