It is important to note how John Donne is actually being quite counter-cultural in this poem. In contrast with other Renaissance songs that idealise women, this song satirises them using hyperbole to underline the point. We can't be completely sure what prompted this rather cynical poem which argues that a true and beautiful woman could not be found, but perhaps we could infer that the speaker is writing after the end of an unhappy romance which might have occasioned the bitterness expressed in this poem.
Bitter it certainly is, for, the speaker assures his hearer, even if a "true" woman could be found, the speaker will still not bother coming to see her because in the time it would take him to arrive she would be unfaithful:
Though she were true, when you met her,
And last, till you write your letter,
False, ere I come, to two, or three.
The cynicism and sardonic bitterness in these last lines that suggest that a "true" woman would betray two or three men in the time it would take to visit her strongly suggests that this is a poem written after the end of a bad relationship that makes the speaker believe that "Nowhere/Lives a woman true, and fair."