This word belongs to the world of being married. Though actually, English almost never uses matrimonial as a name for the married state itself. That name is matrimony, or just marriage. Matrimonial is usually an adjective. It means to do with marriage. You meet it in law and in formal writing: the matrimonial home (the house a couple live in as spouses), matrimonial rights, a matrimonial dispute. A paper might say matrimonial problems instead of they are fighting as a married couple. It is a cold, paper word. Nobody says we are in a matrimonial at dinner. If you want the state of being married, say marriage, or the more churchy word matrimony. There is a rare noun use in legal English; students can leave it.