Sex surrogacy is a very controversial type of sex therapy. The role of the surrogate is to have sex with the client, the purpose being to foster therapeutic gains. The type of sex, and the interpersonal context of that sex, depends on the client's presenting problem. The sex surrogate is typically part of a therapeutic team, working alongside a psychologist, clinical counsellour or sex therapist. The Sessions is a soon-to-be released film about a handicapped man who wants to lose his virginity. Virginity isn't a psychological problem, obviously; however, with the help of his therapist and the support of his priest, he seeks out a sexual surrogate to meet his needs. The film has been getting good reviews (link here).
One comment: After hearing from women who have done sex work, I think it's somewhat problematic that Helen Hunt's character draws a distinction between herself and a "prostitute," as if what she does is more legitimate or commendable. She's still providing sex and intimacy for money, which is nothing to be ashamed of. And if you talk to sex workers, at least the ones that aren't survival sex workers, they'll tell you that many of their clients work through the same sort of issues with their sex worker providers as clients would with their sex surrogates. It's just not in the context of official therapy.