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.
Most sex therapists have a particular awareness of sexuality that rises above personal opinion or personal experiences. We usually have several choices of ways to treat a particular issue when someone presents it. We tailor our treatment to the person(s) before us. We are not a "bigger hammer" there to coerce a person who wants less sex into wanting more. There is a sexological method to treating sexual issues. With the exception of when separate sexual surrogate therapists are added (in a very small number of cases), sex therapy is completely talk therapy.
Sex therapy views sexual issues as being resolved by specifically addressing them, rather than by the assumption that when the individuals in a relationship work out the relationship issues, the sex will just fall into place. For years, I have had a practice full of couples for whom that simply was not true.
Sex therapists also tend to have much greater than average knowledge about the physiological processes that are a part of human sexuality. We tend to work collaboratively with physicians to address the entirety of the causes of sexual concerns.
I would venture to say that there is near unanimity in the sex field when it comes to acceptance of sexual orientations and transgender existence. I have never met a sex therapist who tried to cure homosexuality -- though there are other mental health practitioners who do attempt to do so.
We hold a positive outlook on the beneficial influence that sexuality can have on people's lives and in the world in general. And we are not naive about the ill effects that come as a result of sexuality. We simply try to address those issues from a rigorous scientific perspective, rather than from an ideological perspective.