This is an excellent poem to analyse when we consider Donne's style and talent as a poet. This poem is based around a sophisticated argument where the speaker tries to persuade his lover to sleep with him. She is being "coy" and refusing to have sex with him, and thus the speaker argues that, because they have both been bitten by the same flea, their blood mingles together inside the flea's body. There is nothing wrong with this, the speaker protests:

Thou know'st that this cannot be said
A sin, nor shame, nor loss of maidenhead ;

However, ruefully he sighs that this is "more than we would do." In the second stanza he takes his argument to absurd lengths, actually trying to suggest that the blood that is intermingled in the flea assumes spiritual importance, as he suggests that they are symbolically married now that their blood is together:

This flea is you and I, and this
Our marriage bed, and marriage temple is.
Though parents grudge, and you, we're met,
And cloister'd in these living walls of jet.

Though they have not "met" in reality, inside the flea, they are enjoying an intimacy far more close than they are currently enjoying. So, as they have already "joined," it is nothing for them to join physically now and sleep together. Thus we can see that this poem demonstrates the truth of your statement: John Donne argues in verse, presenting a very sophisticated argument using metaphysical imagery, comparing the flea and their blood in it to a "marriage temple."