[Clare James]

I love Donne. But yes, Anglika's right, you do have to struggle for meaning sometimes.
You might be having trouble with the expression 'to serve your turn' - to be suitable or fitting for your needs.

"Will it not serve your turn to do as did your mothers?" - Why can you be satisfied with inconstancy as your mothers were?'

The main leap of the brain for this poem is to deal with the idea that the poet is asking the woman not to be faithful to him because he doesn't want to be committed to her.