The flea graduates from household pest to religious symbol: what a promotion! The poem has religious overtones all over the place. Though we have the sneaking suspicion that the speaker just wants to have sex with the woman, he tries to argue that he wants to consecrate a holy and sacred religious ritual: marriage, the union of two lives. The flea represents this union because it contains the blood of both of them. He even tries to accuse the woman of attempting not one but three mortal sins when she raises her hand to crush the little bugger.
- Line 4: "Blood" is used both literally and figuratively throughout the poem, which makes it a kind of pun. On the literal level, that flea really does contain two people's blood. But metaphorically, when two people procreate we often talk about "mixing bloodlines," and Donne plays with this double meaning.
- Line 10: The idea that the flea contains three lives is also metaphorical. The speaker thinks of "blood" as a metonym for "essence" or "life." It is a part of a creature that represents another aspect of it. Also, the image of three-in-one alludes to the Holy Trinity, in which the Father, Son, and Holy Spirit are contained in one unity.
- Lines 11-13: Their so-called "marriage" is an extended metaphor that stems from the pun on two kinds of "blood": literal blood and family relations. Mixing of bloodlines is what happens through marriage.
- Lines 14-15: He extends the metaphor even further, saying that neither she nor her parents would approve of the union. The flea is compared to a church or "cloister" with black walls, in which the marriage ceremony takes place.
- Lines 16-18: Returning to the metaphor that the flea contains their lives, the speaker accuses her of trying to commit a mortal sin by killing the flea. She would be murdering him and committing suicide herself. Also, she would desecrate the institution of marriage, by smashing the "marriage temple."