The marital couple is not a dyad but actually a triad because it is defined in terms of the inclusion or exclusion of someone else. This view allows us to see different stages of marriages. In the early part of the marriage the husband and wife are differentiating from their parents. Then they produce children who become part of the marriage. At no point do the husband and wife form a distinct dyad.
Marriages in distress are unstable entities that require an external person to intervene in the dyad to stabilize it, thus making it a triad. A child with a problem can have a stabilizing effect on the marital dyad thus making it a triad. Indeed the marriage counsellor can replace the child and be the stabilizing effect on the dyad of husband and wife. If this happens then the problem of the child ceases. The problem for the therapist is how to exit without the couple destabilizing again and therefore bringing the child in again and the problem returns. In this approach, in any couples counselling there are always at least three people involved in the cause of the marital disharmony.