Not a red rose or a satin heart.
I give you an onion.
It is a moon wrapped in brown paper.
It promises light
like the careful undressing of love.
Here.
The moon goes through cycles every month. This is referring to the hormone cycles that woman experience.  Duffy is openly bi-sexual, so the moon also represents her love for another woman. 
 However this love is cloudy and unclear, being wrapped in brown paper. One possibility is that it deliberately blurs the lines between a heterosexual and homosexual relationship. The poem therefore may have universal application.
Valentine