Deconstruction is a method of analysis that seeks to emphasize the "internal" workings of language and the "assumptions implicit in forms of expression," according to wikipedia. In Donne, this is practically the only way that his verse can be understood, because the relations between ideas and words are nearly always complex and multi-layered. In "The Flea," the insect becomes a symbol of the sexual union of a man and a woman. In this conceit, a rather "low" or even sordid concept is the ironic way in which Donne conveys something that is at the highest level of human experience.
As always in his verse and that of the other metaphysical poets, a dual meaning is expressed: the flea, described as little, which it of course is, is likened to the littleness or seeming triviality of what he wishes a woman to grant to him:
"Mark but this flea, and mark in this, How little that which thou deny'st me is."
He pleads that she not destroy the flea because it not only symbolizes his union with her, but encapsulates three lives: his, hers, and that of the flea itself. Typically, the minutest level of an idea is described in detail; it is as if Donne's mind is always a microscope, inspecting the world and using language on the most basic level to magnify something small or trivial into the expression of a profound idea. By deconstructing, in order to understand his conceptualizations, we are in some sense repeating the same process Donne himself has gone through in creating his verse. The flea symbolizes not only the abstract concept of union, but something more literal:
"This flea is you and I, and this Our marriage bed, and marriage temple is"
In an extension of the metaphor that the flea includes three lives, to kill the flea would then be tantamount to suicide:
"Let not to that, self-murder added be."
But just as the destruction of the flea would be a kind of suicide or murder, Donne seems to reduce the act to one of triviality at the last moment, reiterating that a woman's "yielding her honor" to him is as small a thing as the amount of blood she would lose in having killed the flea:
"Just so much honor, when thou yield'st to me, Will waste, as this flea's death took life from thee."