Sexual relationship counsellor
A sex therapist may be a specialist social worker or psychologist. They advise and counsel in areas of sexual dysfunction, including male erection, difficulty achieving orgasm, relationship problems, issues of sexuality and gender identity, painful intercourse and sex addiction.
Recently, this profession has undergone something of a sea change, some universities offering specialist courses and training. Many people in the news media have also reviewed their (previously scornful) opinion, as therapists have been involved in successfully treating many sex offenders.
They employ a wide variety of methods to improve not just an individual's sex life, but that of couples. Often, counsellors prefer to work with a couple, to improve the chances of success by involving both parties in the healing process.
(In response to a previous writeup): prostitutes are better known in politically correct language as "sex workers" rather than sex therapists.