In "The Flea" John Donne's speaker uses the metaphysical conceit of a flea's blood-sucking to convince a possible lover to join him in physical (sexual) union. It's a kind of pick-up line using very clever and elaborate analogies.
The poem is divided into three parts. Part I is the action of the poem: the flea has sucked the male speaker's blood and the female's. It establishes the literal and sets up the figurative.
Part II is the conceit, or extended metaphor.
This flea is you and I, and this
Our marriage bed, and marriage temple is.
Metaphysical poetry is all about the metaphysical (above, about, or beyond the physical). It takes a literal subject, like a flea, and makes it ephemeral, spiritual, or sexual through elaborate analogy. This comparison is called a metaphysical conceit. So says Enotes:
The extended analogy is also characteristic of this poetry. “The Flea” presents an example of a metaphysical conceit, a type of analogy that requires more elaboration and explanation than other more obvious analogies. Metaphysical poets saw their world in terms of comparisons. Still, even when the similarities between what the flea does with what the couple could do and then the way the flea symbolizes their love can finally be granted, it still remains a strange, if not bizarre comparison. Its outrageousness is part of the effect of the playful pose the poet creates for the speaker. In later and more serious poems, Donne uses the conceit as a way of analyzing his love and his experience of it. Here the conceit makes the poem entertaining and amusing.
The speaker makes the analogy that the flea is a marriage bed and temple. The flea is a holy representation of their physical love. His argument is very clever: since the flea already has both of their blood conjoined, they are effectively married inside the flea. So, the speaker's logic is thus: the woman--since her virginity is already defeated by the flea--might as well go to bed with the speaker. After all, she's already been contaminated.
In Part III, the flea is squished by the nail of the female. This is another conceit that shows the flea's death is a kind of orgasm. The flea's death signals the end of the lovemaking process, which is also a quick rush of fluids, after which both lovers are left weak.
The ending can be read two ways: either the poem (pick up line) worked or it didn't. Either the woman killed the flea in ecstasy or in denial of his advances. I prefer the former reading, although either reading is ironic and humorous.