Marriage was borrowed into English from French in 14c, as was the related verb marry. It denotes both the state of matrimony and the action of getting married, although wedding usually denotes the associated festivities. (Wedding and wedlock are among the words in Old English and Middle English denoting the state of matrimony and the action of getting married, largely replaced in modern English by marriage.)
In contemporary culture there is however considerable dispute about which relationships the word marriage is applied to, especially as regards relationships between people of the same sex. The word’s most frequent collocates are gay and same-sex, followed by arranged, civil, happy, traditional, and interracial. These qualified uses neatly identify the main areas of contemporary debate. The very high frequency of gay and same-sex marriage partly results from hesitation to use marriage unmodified by gay or same-sex to denote same-sex relationships. (Significantly, in verbal constructions the contested area of meaning is less easily flagged: two people of the same sex may be described as celebrating their gay marriage with a gay wedding, but not as being *gaily married.) The verbs of which marriage is most frequently the object are ban, recognize, legalize, arrange, define, and oppose (ahead of propose or celebrate).
Traditional marriage is often regarded as easily defined. But the range of denotation of marriage has always been very broad, and it is difficult to frame a definition which covers all uses. In historical British usage, most uses refer to marriages legitimized (post-Reformation) by the Church of England, since this has been the societal norm. However, marriage has also denoted unions within a huge variety of different religious and cultural traditions, whether those tolerated within religious minorities within Britain, or marriages in other parts of the world. Such uses are not typically marked as problematic or only applying in a limited or metaphorical sense. Formulations such as “so-called marriage” are typically found only where questions of legitimacy within a particular religious or legal context are at stake, although the frequency of the collocation mixed marriage reflects debate about marriages between people of different races or (typically in earlier use) between people from different religious or cultural traditions. In the USA, multiple different local traditions have long applied. One particularly fiercely contested question in 19c concerned the legitimacy of polygamous marriages.
The historically oriented definition of the core ‘matrimony’ sense of marriage in the OED is ‘The condition of being a husband or wife; the relation between persons married to each other; matrimony’, followed by a note ‘The term is now sometimes used with reference to long-term relationships between partners of the same sex’. This is from a revised OED entry published in 2000, but the main definition has changed little from the corresponding fascicle of the first edition, from 1905: ‘The condition of being a husband or wife; the relation between married persons; spousehood, matrimony’. The 1905 definition of the corresponding (transitive) use of marry is ‘To join in wedlock or matrimony; to join for life as husband and wife; to constitute as man and wife according to the laws and customs of a nation’. A significant difference in the (considerably restructured) revised OED entry is that the union is no longer specified as being ‘for life’. This reflects societal and legal changes with regard to divorce. However, perhaps what is most interesting about the 1905 definitions is how inclusive they are: looked at from a philological perspective in 1905, the meanings of marriage and marry were necessarily broad, reflecting the fact that in different nations different laws and customs were found, and the words could be found in unmarked contextual use referring to any of them. It is also interesting that no mention is made of procreation, often invoked in contemporary formulations of traditional marriage, although in Western traditions there has only very rarely been a requirement that children be conceived, although the consummation of marriage by sexual intercourse has often been required.
This long tradition informs current changes in use of the word. Since marriage has long denoted legitimized unions from a wide variety of traditions and jurisdictions, when e.g. same-sex marriage is today legitimized in an increasing number of jurisdictions this reinforces the tendency to use marriage in relation to such unions elsewhere.
Another major factor has also come to prominence recently. The revised OED entry comments concerning be married: ‘Now also sometimes used of couples who are not legally married but whose relationship is recognized as long-term’. Here the semantic development results directly from a change in society. The formal marriage ceremony has become less important for many people. Additionally, some people (often people of the same sex in jurisdictions where they are not permitted to marry) will have a celebration marking their union in many respects identical to a wedding but not resulting in a legally recognized marriage. In the UK many couples of the same sex now mark the beginning of a legally recognized civil partnership in this way.
Both of these seemingly contradictory factors are probably working in the same direction. The meaning of marriage broadens further as a wider range of unions are legitimized in different places, and simultaneously the less obligatory nature of a legal marriage ceremony for what many regard as a marriage leads to still further semantic broadening, even if this results in a mismatch with what is legally recognized as a marriage.
There is probably a further mechanism in the semantic broadening of marriage and related words. Marriage denotes both the state of matrimony and the action of getting married. These two meanings are not easily separated conceptually. The same applies to marry, and although it is not cognate wedding forms part of the same tight semantic field. A wedding marks a marriage, when two people get married, which leads to a marriage, in which two people are married. Where such close semantic relationships exist, broadening in one area is typically reflected by broadening in the others. Hence if a speaker is happy with use of the word marriage to describe the relationship between two people, it is conceptually a very short step to say they are married, hence that at some point they got married, and a party that marked this was their wedding, marking their marriage. In different social circumstances precisely the same semantic links could however work in the opposite direction (this union is not a marriage, hence...). But where a social trend exists for a more liberal interpretation of one of these terms, the close semantic links lead very readily to broadening in the others as well.