This poem is structured around juxtapositions, in particular the contrast between the microscopic flea and the bodies of the two lovers. Contrast in size is extended into contrast in importance, as a couple of flea bites are equated with the mingling body fluids in sex. The woman's casual gesture in killing the flea with her fingernail is then contrasted with the extreme moral scruples that are apparently causing her to postpone losing her virginity.
Things to identify in this poem are the religious vocabulary that Donne employs which give an edge to the poem as its metaphors verge on the blasphemous. Consider this following metaphor:
This flea is you and I, and this
Our marriage bed, and marriage temple is.
The fluids commingling inside the flea are thus compared to a sacred form of marriage--clearly sacrilegious in the time of Donne. The mingling of three bloods with its "walls of jet" is an image of containment, suggesting that the lovers' space is the only essential reality.
However, all of these images are essentially used to get the woman to sleep with the male, authoritative speaker. He says that the amount of honour she will lose in sleeping with him is as trivial as the threat to her life from the flea bite.