Marriage customs in both Westeros and Essos are religious ceremonies. Though they are diverse in their exact customs and traditions, all involve the exchange of vows in the presence of particular sacred witnesses e.g. a septon (for the Faith of the Seven), a heart tree (for the old gods), or a priest or priestess (in case of the Drowned God or R'hllor). Following the ceremony, a feast is held, concluded by the bedding which validates the marriage.
The head of house arranges betrothals for his family members. Although they do occur, marriages to girls who have not yet reached their majority or had their first flowering are rare. Once flowered, girls are seen as fit for bedding. In the Faith of the Seven, a marriage which has not yet been consummated can be set aside by requesting an annulment from the High Septon or a Council of Faith. Once consummated, the marriage is considered to be valid. Though divorce is not common in Westeros, kings in both Westeros and Essos are able to set aside their wives.
Polygamy was practiced in Westeros before the Andals came. The Andals introduced Westeros to the Faith of the Seven, which considers polygamy to be a sin. Regardless, the rule of House Targaryen, who practiced the Valyrian custom of polygamy, was accepted in Westeros, and during the Conquest offers for marriage were made to Aegon I Targaryen even though he already had two wives. Followers of the Drowned God practice a variation of polygamous marriage, as they can take multiple "salt wives" in addition to their "rock wife". Read more...