Many therapists, whether they are willing to admit it to their clients or not, dislike or are uncomfortable with marital and couples therapy. I consider it to be one of my passions, strengths, and specialties. Identifying the underlying dynamics of relationships is key to addressing sexual issues.
Some of the more commons issues I regularly discuss in couples therapy include but are not limited to: sexual discrepancy disorder (when one partner desires more sex than the other), sexless relationships, infidelity, issues with pornography, sexual dissatisfaction, low desire, intimacy issues, lack of communication about sex, polyamory, sex education, loss of attraction, healing after sexual trauma, and considering divorce due to sexual issues.
If any of these issues are affecting your relationship, then couples therapy can be extremely effective.