This article synopsizes the arguments against Prop K so well! One thing to add, Prop K wouldn't just "bring the job title of ‘prostitute’ into a legitimate labor category in San Francisco" it would also do the same for pimps.
The author of Prop K and the head of the "union" that sex workers would belong to should Prop K pass, defines pimps as support staff and has written: "Our support staff, which we reserve the right to contract, have long been demonized as 'pimps'"
How can empowering the oppressor EVER lead to better conditions for the oppressed???