Its interesting how marriage has been redefined a number of times in the past, not just in 2013 with our equal marriage legislation:
Prior to 1856 boys could get married at the age of 14 and girls at 12 (in Scotland at least).
Prior to 1907 a man could not marry his deceased wives sister. This was changed by the Deceased Wife's Sister's Marriage Act 1907.
Prior to 2006 (in Scotland) it was still possible to form a marriage by "cohabitation with repute". Some of the rules for defining this in law were odd things like whether your clothes were washed together at the same time, whether you ate together etc.
Prior to 1967 in the USA interracial marriage was illegal in many states.
Prior to 1857 in England divorce was not commonly available. This was changed by the 1857 the Matrimonial Causes Act.
In 1836 the marriage laws in the UK were changed to allow non religious marriages in registry offices.
In any case, prior to 1560 (in Scotland) it was almost unheard of for people to get married formally in church unless they were people of land and title.
Oh and Mary and Joseph were not married at the time of Jesus birth. They were "betrothed". In Scotland we had a similar thing called handfasting which was a trial marriage that lasted for one year:
"It was an ancient custom in the Isles that a man take a maid as his wife and keep her for the space of a year without marrying her; and if she pleased him all the while, he married her at the end of the year and legitimatised her children; but if he did not love her, he returned her to her parents." (Màrtainn MacGilleMhàrtainn, A Description of the Western Isles of Scotland, 1703)
Oh how the times have changed!