A sex therapist can be a psychiatrist, a marriage and family therapist, a psychologist, or a clinical social worker. We are specially trained in sex therapy methods beyond the minimal amount of training about sexuality that is required for each of those licenses.
There are a few graduate schools in the U.S. that specialize in training for sex therapy.
Some people assemble their training by rigorous self-study and by
attendance at the major sexological organizations' annual conferences.
We have about a dozen scientific journals dedicated solely to sexual
research. There are about six major organizations that hold conferences
and trainings.
So seeing a sex therapist is like going to a gynecologist
for gynecological problems rather than to a family practice physician.
Both have specialized particularly in that area. That isn't to say that
one couldn't get good help from a non-sex therapist for a sexual issue,
it's just that the likelihood might be a bit less.