By day she wooes me, soft, exceeding fair:
 But all night as the moon so changeth she;
 Loathsome and foul with hideous leprosy,
And subtle serpents gliding in her hair.
By day she wooes me to the outer air,
 Ripe fruits, sweet flowers, and full satiety:
 But through the night, a beast she grins at me,
This alludes to Medusa, the Gorgon of Greek mythology, whose ill-treatment by a man resulted in her transformation to a serpent-haired monster who could kill with a glance. In her ‘The World’s Wife’ collection, Carol Ann Duffy re-cast this story in kinder terms from the wronged  Medusa’s  point of view. 
 It also alludes to the serpant in  Genesis 3:1 , the representation of the devil who ensnares Eve first, before she urges Adam to sin. 
 Note the sinister, hissing  alliterative  ’s’s in ‘subtle serpent.
The World