Explication on The Flea

In this poem, the "I" of the poem is lying in bed with his lover, and trying to get her to give her virginity to him. (It could, of course, quite easily be a female "I" trying to seduce a male, but I will stick with one for convenience). While lying there, he notices a flea, which has obviously bitten them both. Since the 17-century idea was of sex as a "mingling of the blood", he realizes that by mixing their bloods together in its body, the flea has done what she didn't dare to do. Then, he argues, since the flea has done it, why shouldn't they? To back up his argument, he refers to the marriage ceremony, which states "man and woman shall be one flesh". He argues that since they have mingled their bloods and are therefore "one blood", they are practically "one flesh" and are therefore married! Not only does that reinforce his seduction argument, but it also provides ammunition for him to defend himself when the female does the next logical thing and moves to kill the flea. Donne argues that by spilling his blood and hers by killing the flea, she is practically committing murder. Not only that, but by breaking the holy bond of marriage she is committing sacrilege!
However, the flea finally is killed, and the poet is forced to change tactics. There, he argues, killing the flea was easy, and as you say it hasn't harmed us - well, yielding to me will be just as easy and painless.
This poem borrows a lot of religious imagery, because it helps add an aburd authority to the poem, as Donne tries to argue that what they are about to do is not only supported by God, but to not do it would be heretical. Some literary devices used in this poem include; direct address: Marke but this flea... and repetition: And marke in this.