This little poem is a marvel of form and rhythm. Donne makes the writing look so easy that you hardly notice everything going on beneath the surface.
Let's start with the rhyme scheme: AABBCCDDD. These couplets (and one triplet at the end of the stanza) help you keep track of the speaker's argument, which generally proceeds in two-line units. So each time we get a new rhyme, we're also getting a new idea. The rhyme words are very simple, usually limited to one syllable: this/is, thou/now, met/jet. The most commonly used rhyme words are "thee" and "be." Notice, too, Donne's clever pairing of "me," "thee," and "be" at the end of the poem. He manages to unite the couple in rhyme, if not in real life.
The poem's main rhythmic unit is the iamb: a short, unaccented syllable followed by a long, accented syllable:
This flea is you and I, and this Our mar-riage bed, and mar-riage tem-ple is (lines 12-13)
The lines alternate between eight and ten syllables (iambic tetrameter and iambic pentameter). Each stanza has nine lines, and the first and last line of each stanza has eight syllables.