Not long after her conservative colleagues raised this argument, however, Justice Ruth Bader Ginsburg explained exactly why marriage was long understood to be incompatible with homosexuality in just five sentences:
[Same-sex couples] wouldn't be asking for this relief if the law of marriage was what it was a millennium ago. I mean, it wasn't possible. Same-sex unions would not have opted into the pattern of marriage, which was a relationship, a dominant and a subordinate relationship. Yes, it was marriage between a man and a woman, but the man decided where the couple would be domiciled; it was her obligation to follow him.
There was a change in the institution of marriage to make it egalitarian when it wasn’t egalitarian. And same-sex unions wouldn't — wouldn't fit into what marriage was once.
Justice Ginsburg’s point was that, until surprisingly recently, the legal institution of marriage was defined in terms of gender roles. According to Sir William Blackstone, an eighteenth century English jurist whose works are still frequently cited today to explain the common law principles we inherited from our former colonial rulers, “the very being or legal existence of the woman is suspended during the marriage, or at least is incorporated and consolidated into that of the husband; under whose wing, protection and cover, she performs everything.” As late as 1887, fully one third of the states did not permit women to control their earnings. And married women could not even withhold consent to sex with their husband until shockingly recently.
Under the common law, “by their mutual matrimonial consent and contract the wife hath given herself up in this kind unto her husband,” and this consent was something “she cannot retract.” The first successful prosecution in the United States of a husband who raped his wife did not occur until the late 1970s.
So American marriage law, and the English law that it was derived from, presumed that the wife was both financially and sexual subservient to the husband. In a world where marriage is defined as a union between a dominant man and a submissive woman, each fulfilling unique gender roles, the case for marriage discrimination is clear. How can both the dominant male role and the submissive female role be carried out in a marital union if the union does not include one man and one woman? This, according to Justice Ginsburg, is why marriage was understood to exclude same-sex couples for so many centuries.
But marriage is no longer bound to antiquated gender roles. And when those gender roles are removed, the case for marriage discrimination breaks down.
Generally speaking, any argument based on "we've always done it this way" seldom works. If "we've always done it this way" – it was some sort of rule we had to follow - we'd still have slavery, women would still not be permitted to vote, businesses could still post "No Irish Need Apply" signs - and yet there would still be separate drinking fountains for different races of people. In fact, it would be difficult to get any new laws passed at all.