I think it's rather odd that therapy isn't covered. I guess I've got a good plan with mental health benefits, I don't even think I have to specify what kind of therapy I'm undergoing; therapy is therapy, whether for marriage problems or depression or what-have-you.
That being said, sometimes you can find low-cost or free counseling through community groups or churches. I don't necessarily think you have to look only for marriage therapy per se - as long as whatever counselor you work with has experience with relationships. Lots of pastors do counseling, and there are some forward thinking ones - perhaps look for Unitarian or Reformed churches. Google your area for women's groups, too. Maybe you can meet someone who knows a resource. You can also see if there are any programs run by your town or county. All the best to you!