Marriage has never had just one meaning. Adjectives commonly used with the word reveal the institution’s diversity, among them traditional, religious, civil, arranged, gay, plural, group, open, heterosexual, common-law, interracial, same-sex, polygamous, and monogamous. And this diversity has been in evidence, if not since the beginning of time, at least since the beginning of marriage itself, roughly some 4000 years ago.
Multiple wives, for example, proliferate in the Bible. King Solomon famously had 700, although most were apparently instruments of political alliance rather than participants in royal romance. (For that, he had 300 concubines.)
Marriage can be sanctioned legally or religiously, and typically confers upon married people a special legal status with particular rights, benefits, and obligations. Access to this special status has changed over time. For example, the U.S. Supreme Court legalized interracial marriage as recently as 1967, while same-sex marriage, which for some time had been banned in many states or ignored in others, was in 2015 ruled a constitutional right for all Americans.
Marriage as the union of one man and one woman is the most common definition of the term in the Western world today—this in spite of the prevalence on the one hand of divorce (enabling people to marry several different partners in sequence), and on the other, of an increasing acceptance of same-sex marriage. And as society becomes more inclusive, it is likely that “equal protection under the law” will be fully applied to same-sex couples.
In crafting definitions for a word that represents an institution that is rapidly evolving, the dictionary may well have to keep adding, changing, and reordering senses, splitting or combining them as the institution changes. Inevitably, those who want to preserve what they cherish as traditional values will resist new definitions, while those who anticipate, welcome, and fight for societal change will be impatient when new definitions do not appear as quickly as they would wish. But we should all remember that while it is not the job of a dictionary to drive social change, it is inevitable that it will reflect such change.