What is the theme in The Happy Hooker by Xaviera Hollander?

Xaviera repeatedly argues that prostitution is as legitimate a profession as any, citing instances in which she has helped men with sexual dysfunction, sometimes even at the request of a psychologist, in addition to the services she provides normal customers. Xaviera is outraged at the fact that prostitution is illegal in the United States and that prostitutes are persecuted, having to pay bribes and bail to stay in business.