One of the more prevalent arguments waged against marriage equality is historically equivalent to creationism. Creationists ignore science and argue, based entirely on their reading of Bible mythology, that the world is 7,000 years old and that species don't evolve. For them, reality has to fit their theology, not the other way around.
One of our projects at the Moorfield Storey Institute has been the fight for marriage equality. To do that, I've had to do something that opponents of equality fail to do: research. I've read a dozen or so decent histories of marriage, countless papers on the situation regarding the laws, and contemporary looks at what really does happen when marriage equality is realized.
In that study I realized that marriage and the origins of life are similar. There are two basic views. One assumes that marriage was created pretty much as it supposedly existed in 1950s America. There was a husband, who was a wage-earner, with a stay-at-home wife and 2.1 children. For good measure, there was a dog, a cat, and grandparents who provided babysitting when Mom and Dad had to attend a business dinner.
Of course, actual marriage in the 1950s wasn't even like that, but then marriage has never been like that. Faith-based history is different. Focus on the Family claims that "God created marriage as a loyal partnership between one man and one woman." The Protestant Reformed Churches of America, a Calvinist outfit, claims that "God created the family in paradise as the first institution He made." They are also quick to tell you that "woman is not man's equal (except in terms of their salvation)."
The creationist view of marriage is that a divine being magically made marriage appear one day by creating a man and a woman and having them make a family (which always raises the question as to the sexual partners of the children of that first couple, especially as the Biblical account has Eve giving birth only to sons).
However, not even the Old Testament verifies what the creationists argue. It showed marriages to be anything but the Focus on the Family version of a loving couple and their loving children. The men of the Old Testament, including patriarchs of the Bible, often had multiple wives. Abraham supposedly had relationships with Sarah and Hagar. If the Book of 1st Kings is to be believed, Solomon had 700 wives and 300 concubines -- which really does sound like male bravado more than anything else.
When you look at the real-world history of marriage, you don't see any evidence for this creationist view. What you see is that marriage evolved and has kept evolving. At different times, in different places, marriage filled different functions. The functions marriage filled determined the nature of the marriage at the time. Form follows function.
When land was the major means of production, marriage was intimately tied to the acquisition of land. Under Old Testament law, a man was obligated to marry his brother's widow, as it helped keep land in the family. Where land was a prime consideration, marriages were often arranged, and marrying close kin was common.
In Imperial Rome, families that ruled the city were very concerned with the politics of marriage. Alliances were formed or dissolved through marriages and divorces. Meanwhile, the husband wasn't expected to necessarily love his wife, or desire her. No one was horrified if he stepped out on her with a mistress, or even a male lover, provided that rules meant to preserve the social hierarchy were followed -- at least in public.
Marriage, at other times, was primarily about household production. The natives of North America had male functions and female functions. Various tribes allowed same-sex relationships, provided that one of the partners took on the role assigned to females. In addition, they often ascribed mystical powers to this individual as a "two-spirit" being, encompassing the spirits of both genders.
For most of human history, marriage was not about love at all. The idea of marriage being about loving couples is very much a modern idea. Of course, once love and mutual nurturing became a prime function of marriage, it opened the door to same-sex marriages, as well.
Rome's ruling families saw marriage as politics, not love. Marriage was to insure that a political elite held power. Romance was found outside marriage. Sexual satisfaction could be found there, as well, with partners of either sex, with little disapproval. A homosexual Roman gentleman could still have his romantic interest, because that was not the function of marriage.
The moment someone tells me "marriage has always been" something or another, I know they are ignorant of the actual history of marriage. It has never "always" been anything. It has taken different forms, with different social rules attached. Those forms and rules changed as the function of marriage changed.
In our world, the function of marriage is not about land, politics, or even procreation. It is about love and mutual support between the spouses. Given the modern function of marriage, it is hardly surprising that gay couples want to be included. They have the same needs and wants as other couples, and marriage provides them the same benefits and serve the same functions as it does for straight couples.
Marriage is not a stagnant institution. It is a vital, evolutionary institution constantly changing forms as the functions it fills adapt to new circumstances.