I have had both lesbian and straight therapists who I had positive therapeutic relationships with. But I do have to admit, I found it much easier with the lesbian therapist, simply because I found I did not have to provide as much context/ translate as much of my life experience for her as I did for the straight therapist....

But I am sure it depends a little on the issues you are trying to address in therapy. If I was trying to work on relationship (love relationship) topics, I would definitely prefer a lesbian therapist. For childhood topics etc I found it to be of less importance.