Oxidoreductase involved in lignan biosynthesis. Catalyzes the stereospecific conversion of (-)-secoisolariciresinol to (-)-matairesinol via a lactol intermediate. (-)-secoisolariciresinol + 2 NAD(+) = (-)-matairesinol + 2 H(+) + 2 NADH Homotetramer. Belongs to the short-chain dehydrogenases/reductases (SDR) family.