It is surprising to the sexdoc that he receives about one question a week asking how to become a sex therapist, and that more than half include some reference to "I doubt you ever get this question, but ..." So instead of answering individually, here's the "public" answer. This is my opinion; I strongly urge you to get at least several.

I strongly endorse and encourage the use of the "research interview". If you are considering a particular vocation, ask someone now doing that job for time to discuss that with you. The stressed-out over-worked people who have difficulty managing time probably won't do it (that should tell you something).

The short answer is that the most traditional route is to become a therapist first, then specialize in sex therapy second. Most states have licensing for social work and for psychology. With a Master's degree in Social Work (MSW) specializing in Clinical Social Work, some states allow you to become an LCSW: Licensed Clinical Social Worker. With a Master's in Psychology you get something like (what they do in California:)  LMFCC:  Licensed Marriage, Family, and Child Counselor. Note: Master of Arts requires a thesis. Master of Science usually doesn't.

At the doctoral level, you earn a Ph.D. in Psychology, Educational Psychology, Counselling Psychology, Clinical Psychology, or Education. All of these (if obtained from bona fide schools, not mail order houses) require a doctoral dissertation (small book) that "proves" that you know how to conduct research. To get a doctorate in psychology without doing a dissertation you get a Psy. D. degree

In California, whether at the Master's or Doctoral levels, you have to have 3,000 supervised clinical contact hours, not more than 1,500 of which were obtained pre-degree (translation: at least 1,500 of which must be earned post-degree).

The next step varies from state to state but usually you submit your verification of hours and your coursework to a governing body, and if they deem that you have the necessary qualifications, you are permitted to sit for a written exam (again, varies dramatically from state to state). When you pass (many people take it every six months for three years before passing) you are then permitted to schedule the oral exam. When you pass that (it is not uncommon to take it several times before you pass), you get licensed. Many states aggressively prosecute people who practice psychology (psychotherapy) without a license, but there are loopholes.

The route I have just described pretty much requires experience in the broad spectrum of psychological problems. You are trained to deal with depression. obsessive-compulsive behavior, alcohol, drug, and other addictive problems, suicide, adolescent acting out, etc. Then if you want to specialize in sex therapy you have two basic approaches. One is to go to a proprietary school -- for profit -- for classes and supervised experience. The other is to apprentice under a practicing sex therapist and to read, read, read, and read some more. Your supervisor can individually tailor your experiences and resiliently help you understand your mistakes (which we all made and make) as you practice on patients -- that's why we call it the practice of psychology, not the science.

Some people go straight from college into a proprietary school to obtain a Doctorate in Sexology.  I am not sure about other states, but in California practicing Sexology is not regulated, and is not prosecuted so long as the clinician limits therapy strictly to sexual problems. I do not know of any insurance company that will reimburse for the services of a sexologist.

My bias: I know some great sexologists, but as a general matter, I think that it is critical that a good sex therapist be proficient in diagnosing all the psychological problems. Sometimes diminished sex drive is just apathy, but sometimes -- indeed, often -- it is a symptom of other problems, such as depression. Probably 60% of what I do is treat underlying problems along with the sexual matter.

Time investment: After high school: 4 years of college, 2 years for a Master's degree. If you go on, the average is 7 years to complete a Ph.D. Then, how fast can you accumulate another 1,500+ hours of supervised clinical contact (with patients)? At 20 a week (heavy caseload for a neophyte) that's 75 weeks.

Questions? Please let me know.