Check out today's poem from Jellyfish Magazine: "Marry Me" by Lindsey Webb.
Now write a poem in which the speaker tells someone why they cannot possibly accept a marriage proposal.
Your speaker can be addressing the person who proposed or a confidante. Your speaker can be reacting in the present to the question just asked, or can be explaining a past decision.
Either way, be both specific and strange (again, see today's blog poem).