Depends on what you mean by "sex"

It’s a question of statutory interpretation, not contract interpretation, but it’s interesting anyway. Does a state law defining a prostitute as one "who engages or agrees or offers to engage in sexual conduct with another person in return for a fee," cover actors paid to engage in sex in the process of making pornographic films?

No, according to New York Supreme Court Justice Budd Goodman. The essence of prostitution, he concludes, is a bilateral arrangement where A pays B for sex with A. If A pays B to have sex with C, so that A can make a ton of money, it’s not prostitution, it’s a legitimate business.