One can make the case, however, that Medusa did not enter into her relationship with Poseidon willingly. Another version of the story suggests that Poseidon used Medusa to humiliate Athena. To do this, the sea god violated Medusa in Athena's temple. This telling of the myth also does well to note that Medusa reached out to Athena, both for guidance and forgiveness. 
What this version shows is that Medusa did not willingly break her vow to Athena, which only further attests to the idea that her relationship with Poseidon was not consensual.