There are many important considerations to weigh when choosing a therapist.  There are two very useful pamphlets published by the American Psychological Association.  They are: A Mental Health Patient's Bill of Rights, and, Change Your Mind About Mental Health.

One obvious consideration when choosing a therapist is gender: do you have a preference for a male or a female therapist?  For many people, the gender of the therapist is of little concern.  For others, this is a very important consideration.

While many women assume they will feel more comfortable talking to another woman, it is important to consider the benefit of developing a healthy therapeutic relationship with a supportive, accepting, nurturing male.

Experiencing the safety of such a relationship, in which appropriate boundaries are clearly defined and respected, can help a woman redefine and establish more healthy relationships with men in other areas of their lives.

For heterosexual couples choosing a marriage counselor, one fact to consider is that there will be either two women and one man, or two men and one woman in the therapists' office during counseling.  This should not make a difference to the progress of the marriage counseling, but may affect the perception of counseling during the first few sessions.  Especially if one of the partners in the couple does not really want to go to marriage counseling, that person's perception might be that they are being "ganged up on."  This "two against one" perception can be overcome if the therapist is the same gender as that person.

In general this is not usually a problem, and when it is, it is commonly experienced during the first few sessions only, until trust can be established, and a familiarity with the process, and with the therapist, can be gained.

http://www.apa.org/pubinfo/rights/rights.html