Marriage is an institution that, by natural law (binding on all human beings) and divine revelation (binding on believers), can exist only between one man and one woman. Everything else one says about marriage, no matter who says it, including about the purposes of marriage, the duration of marriage, the religious and/or civil requirements and ceremonies for entering and ending marriage, the sacramentality of certain marriages, some apparent historical anomalies among the foregoing, the question of marriage nullity, everything else about marriage assumes the man-woman relationship.