The flea is the two lovers because it contains a piece of both of them, it is the marriage bed because it is the first meeting of their bodily fluids, and it is their marriage temple because it is where their relationship is sealed. Donne goes on to plead the flea's case as he continues to use metaphors to show how terrible it would be if the flea died. The girl would be killing the male, committing suicide, as well as sacrilege, simply by crushing this one little flea.
In the third stanza the women finally kills the flea and the argument of how terrible it was to kill the flea seems to be discredited. The metaphor continues to grow more absurd when in line 20 an allusion to Jesus being killed on the cross is related to the female shedding the "blood of innocence". The point is then made that all this time the narrator spoke of how tragic the death of this flea would be and once it was done neither of them were any worse off. Just as it seems that the whole argument has fell apart the narrator uses this fact to back his original argument by telling his female companion in line 25 to "learn how false fears be". Just as it seemed to be so terrible to kill this flea, once it was finally done they both were fine, just like how it would be if the virginity of the female were taken.
It is obvious that this poem is supposed to be humorous and not taken as a legitimate argument for the narrator to have intercourse with the female. Regardless of this, Donne takes seemingly absurd metaphors; such as flea the is sex, the lovers, the trinity, etc., and makes them an entertaining poem of one mans plight to have intercourse. It goes a long way to show, and help the reader understand, the absurdity of what a man thinks about by using metaphors that you wouldn't think work and laying them out in a way that they are very effective and even logical.
Another example of Donne effectively using unrelated objects as metaphors is in the poem "A Valediction: Forbidding Mourning".
