The purpose of sex should be defined by the participants, not by third parties. Government (public, third party) interest in the private act should be limited to protecting the participants from fraud, undue influence, unexpected diseases, unexpected pregnancies, violence... Basically, so long as everyone involved in the act is aware and freely agrees to the purpose, the actual purpose is functionally irrelevant.

Sex work is work.

I can see regulations against establishing an employer/employee working relationship, and instead mandating a private contractor relationship: an employee can disciplined or fired for failure to perform, or for refusing to serve the employer's customers; a contractor can pick and choose their own clients.

The right of assembly; the freedom of association should broadly protect mutually consensual activities.