Mark but this flea, and mark in this
How little that which thou deniest me is;
It suck'd me first, and now sucks thee
And in this flea our two bloods mingled be.
Thou know'st that this cannot be said
A sin, nor shame, nor loss of maidenhead;
Yet this enjoys before it woo,
And pamper'd swells with one blood made of two;
And this, alas! is more than we would do.
O stay, three lives in one flea spare,
Where we almost, yea, more than married are.
This flea is you and I, and this
Our marriage bed, and marriage temple is,
This line begins with an exclamative “O stay”, suggesting that she is unresponsive to his arguments, and that his desperation is growing. 
 Blood is an obvious  metonymic  symbol for life. In a time before the circulatory system was understood, the soul or spirit was believed to move the blood around the body: thus, blood contains the spirit and life of its owner. 
 Donne is referencing the Holy  Trinity  as three ‘lives’ are in one: Father, Son, and Holy Spirit. The speaker is evoking religious language to support his argument. 
 Three is a repeated symbol in this poem: there are  three  stanzas, each of nine lines ( 3x3 ).
The Flea