Isn´t it somewhat incongruous to legalise prostitution and have such strict sexual harassment laws that a boss that offered workers a raise for performing sex acts on him/her would be prosecuted?
Not at all. Your boss is your boss; your client, your client. The two are mutually exclusive for the employee, and there is no contradiction inherent or implied. The nature of the business has no bearing on the strength of the law.
Besides, you obviously don't know how the workers' wages are calculated. At least, here in the U.S.
There is a set price, which is called a "menu" in most houses, but the ladies often haggle with the customer and get still more money, if they're smart. Trust me, most of them are more than smart. But the boss wouldn't negotiate in any fashion with individual workers in his house. What one makes, base price, they all make. There are no individual salaries paid by the employer. Whatever price the worker negotiates with her client, the house takes half. Period.
In Nevada, where prostitution is legal in certain counties and is closely regulated, no one is
ever forced. The ladies have full right to refuse any client for any reason. It's no big thing; if it's a matter of personal taste, the client has plenty of other ladies from which to choose. If it's a matter of safety, the house fully supports its staff, and for a sex-worker to get hurt on the job is incredibly rare and not tolerated.
My information for this comes from my friend, who has been a worker in a house in Nevada for over 5 years, and from my own, separate reaserch, which last is available to anyone who cares to look for it, rather than make up hysterical hypotheticals from whole cloth.