Marriage is a sacred institution. Marriage was developed by societies as a means of ensuring the generally acknwledged (at the time) most effective means of maintaining an orderly and productive society: by encouraging, rewarding and supporting the most basic building block of these societies, families with children. Gender roles were more strictly defined in earlier times, and therefore the likelihood of successful families were increased when there was a man and woman who remained together to raise children. This concept of a nuclear family as the basic unit of a society is much older than democracy; in the ancient cultures where the concept of marriage was developed, the ruling class was often a religious class and bestowed the blessings of marriage upon couples with the hope that they would be prosperous and fruitful in terms of perpetuating the culture through their industry and having children. Civil Union is not the same as marriage; this is a legal arrangement bestowed by the secular government allowing legal rights including accession of property, decision-making rights, and the like. All of the rights of marriage can be developed through other legal arrangements and, to some people, should be made in this way independent of the concept of marriage. In many countries, they aren't anymore. Many governments do (and should) view marriage as a legal financial/medical/property agreement; this especially should be true if there is a separation of church and state. In many countries, a couple may choose to have a religious ceremony performed, but they still must file papers with the proper state/country authorities to have the marriage recognized as valid. For deeply religious people, marriage may be seen as part of a covenant, a spiritual binding before their god. Historically, the prevailing religion in a country would had have domain over family matters such as announcing of banns (engagement), marriage ceremonies, recording of children, deaths, etc. Marriage symbolizes the union of a man and a woman coming together in the eyes of their creator and to embrace the teachings of their creator and envelop those teachings into their daily lives. The way I see it, until recent centuries marriage was mainly for the purpose of reproduction. As far as history dates back, reproduction ties in very closely with spirituality and/or religion. I imagine that this has to do with the mysteries of life such as our origins, the wellness of our newborns, death, generations, family, prosperity, etc. But I am no expert - this is only a personal conjecture.