Dear Christians who oppose “gay marriage” because it isn't "Biblical marriage",
There may be a lot of reasons that you're opposing “gay marriage”/marriage equality. But your alleged support of “Biblical marriage” isn't one of them. I'm not saying the Bible isn't the reason you oppose gay rights. I'm saying you do not support “Biblical marriage”.
It might be a popular line – more popular, at least, than “I think gay people are abominations, because the book that I ignore when it affects things that I do, like wearing mixed fabrics, eating bacon, shaving sideburns, etc., says so; therefore I don't think they should have the same rights that I do” – but it's not true. And if you're about to counter with, “Oh, but God created marriage to be between a man and a woman!” or “The Bible defines marriage as a union between a man and a woman!” save it. Seriously. That a) is not true and b) doesn't matter anyway.
As to the a. Abrahamic faiths don't get to define marriage. That's a human-defined state that existed long before Christianity, Judaism or Islam. When Abraham – the Abraham of “Abrahamic” – visited Egypt, they already had (and had long had) the concept of marriage. It has existed throughout time in almost all cultures.
But it's not simply a question of marriage predating the Bible – and this is perhaps the larger point. The fact is, the Bible does not define marriage the way socially conservative American Christians pretend it does. The “traditional marriage” – a man and a woman (and often enough his mistresses) – that we talk about is “traditional” in the western world for the last 2,000 years or so. But Biblical it is not.
Biblical marriage is a man arranging to buy a girl from her father for an agreed upon purchase price (Genesis 29:18).
Biblical marriage is a wife “giving”, regardless of her maid servant's wishes, her servant to her husband as a “wife” for sex and procreation (Genesis 16:2-3, Genesis 30:3, Genesis 30:9, etc.)
Biblical marriage is a raiding party murdering the fathers and mothers and brothers and sisters of a people but saving the young virgins because they want “wives” [i.e. women to capture and rape, legally] (Judges 21:10-14)
Biblical marriage is a raiding party lying in wait to capture more women as “wives” to legally rape (Judges 21:20-24)
Biblical marriage is a victim being forced to marry her rapist with no hope of divorce [but don't worry – her father is suitably compensated in cash for the trouble, and this is only valid if the woman is not already another man's property...so relax, no property rights are violated by this arrangement!] (Deuteronomy 22:28-29)
Biblical marriage is selling your daughter as a slave to be given to her owner or owner's son for sexual exploitation as a “wife” [though denied the minimal protections of a wife] (Exodus 21:7-11)
Biblical marriage is one man taking multiple, even hundreds, of wives and concubines (see: David, Solomon, Jacob, Abraham, etc.)
Biblical marriage is a woman as property whose own happiness is inconsequential, but whose property status is absolute (see: David and Michal)
When it comes right down to it, Biblical marriage is two or more men deciding between themselves what woman an individual will take as a wife – be it a father selling his daughter into sexual slavery, a husband-to-be arranging with a father an agreement suitable to both parties (irrespective of the wife-to-be's wishes) on how to dispose of/acquire the female in question, a party of raiders murdering a woman's entire family in order to claim her without any resistance, a rapist grabbing an unattached female and at the same time getting himself a new wife, etc., etc. Biblical marriage isn't love and romance and butterflies in the pit of your stomach. Sure, that probably happened sometimes. Not because of, but rather in spite of, the institution as it was practiced. And in the other cases, as the story, for instance, of the acquisition of wives for the children of Benjamin (Judges 21), one can only assume it entailed a lifetime of misery (wiping out a woman's entire family to force your sexual advances upon her rarely entails good will; and such was the plight of 400 women in that case, and many more in other stories).
Very few American Christians nowadays embrace true Biblical marriage, and most would not dare to admit it openly – because most facets of it are illegal! They throw the term around a lot, because it sounds nice and quaint and moral, but they don't truly support Biblical marriage. People want to fall in love, marry the person they love, and raise kids who will do the same. No one wants their father, or their sweetheart's father, pimping them out to the highest bidder (pardon me, “slave owner/husband-to-be”). They don't want their daughters (or selves) to be grabbed, raped and “married” for life to whatever lonely pervert happens to be skulking by. They don't want guys to be able to group up, grab some weapons, massacre entire families and claim “wives” in the process. Nowadays, we put the perverts in jail; we don't give them young women whom they've abused. Nowadays, we'd put the marauders in jail and sometimes kill them; we don't give them their prey as “wives”. Nowadays, slavery is illegal, and we try to destroy the sex slave trade (not allow it to legally flourish so long as men make the victims their “wives”). And what's more, nobody is advocating to go back to these good old bad ways.
So, no, you do not support Biblical marriage. You support a tiny, tiny subset of it, reshaped and influenced by thousands of years of western culture and enlightenment thinking about the rights of women. And those tiny little subsets? Also found in cultures around the world, predating Christianity and the Bible. You could just as easily say that you support traditional, ancient Egyptian marriage as Biblical marriage – because you keep about the same from either of them: the idea of a legal construct that binds people together. Not the number of people involved, not the rights and expectations associated, not the rules that govern it. Just the most basic concept of a legal joining. But that is not Biblical marriage.
All of which leads to my second point. It. Doesn't. Matter. There's this thing called the Constitution. And that guarantees that you can believe whatever you like – so even if you want to believe that the mutant, scarcely recognizable remnant of “Biblical marriage” that you've scraped from the pages of the Bible is in some way pure Biblical marriage, you're absolutely free to do so: you can ignore every facet of actual Biblical marriage except the one that it doesn't actually mention (“gay marriage”). Furthermore, you can extend that belief into your own life, and never be gay married. But the flip side of constitutional protection is this: it also protects the country from your beliefs being forced on it. Believing that your religion teaches something, regardless of whether it does or not, is not reason for it to be law. So you can support, and practice, “Biblical marriage” – not the real kind, because that will lead to incarceration and even, potentially, finding yourself on the wrong side of the death penalty. Sorry. I mean the fake kind that Christian leaders and politicians peddle as “Biblical marriage” (also known by the more accurate, but still not reason-enough-to-legislate moniker, “traditional marriage”). But you can't keep gay people from being married on the strength of “but God invented marriage to be just like it is, today, in the United States”. Because your belief in what God did or didn't do is irrelevant to the rights of other people. Thus saith the Law of the Land.