The argument would be that criminalizing the act of prostitution itself - if legislated - would be an infringement on the "life, liberty and security of the person" provision of the constitution (s. 7) because it would increase the jeopardy to the girls by preventing them from accessing law enforcement assistance when beaten or raped and this infringement is not "saved" constitutionally by the negligible benefit to society of criminalizing a victimless crime.
The risk substantially outweighs the arguable and slight benefit.