It's my understanding that qualifited sex therapists have also had training in marital counseling and some amount of psychotherapy. The trick, of course, is finding one who best suits you. I suggest going for an initial interview with more than one and ask questions. I think some of them will give you a free first session to help you make that determination.
Remember that web sites, especially like Marriage Builders, have a very specific philosophy and solution approach. It apparently works for many of the people who choose the site, but that's no guarantee it's the best approach for you.
