Historic alterations in the customs and protocols of marriage give rise to the argument that marriage is dynamic and same-gender acceptance is the latest evolution of marriage. Some societies have from ancient times permitted spouses to have multiple concurrent marriages while many societies discourage this practice today. Having more than one marriage at the same time is called polygamy for men and polyandry for women. Marriages that involve more than two people at one time in a single act of matrimony are considered to be a "group marriage" and are even more rare.