This is a common and often misunderstood question! Let us help clear it up. :)
In order to bill insurance of any kind, there needs to be a diagnosis that demonstrates the service we provide is "medically necessary" to treat the problem. For example, if we conduct an assessment and determine that someone has generalized anxiety disorder, we can request payment from an insurance company for providing therapy because that service then becomes a payable treatment we provided for a diagnosed problem.
Couples counseling, however, often focuses on improving relationship dynamics, communication, and conflict resolution, rather than a diagnosed mental health disorder. Although highly important to resolve, problems with interpersonal relationships aren't medically necessary and therefore, insurance will not pay for treatment.