In the same way that we look back on the arguments for slavery or denying women the right to vote, will we come to view the ethical arguments against same-sex marriage as laughable? Michael Bradley writes.

Just for a moment, assume that not everyone who opposes same-sex marriage is homophobic, that racists don't necessarily hate black people and that sexism and misogyny aren't the same thing. And let's explore what that means.

"The great moral lesson of Othello is that black and white blood cannot be intermingled without a gross outrage upon the law of Nature." That was John Quincy Adams, sixth president of the United States and passionate advocate of the human rights of African slaves, writing in 1835 on the subject of intermarriage between blacks and whites. Not overly surprising; his mother Abigail, wife of the second president John Adams, saw Shakespeare's play in 1786 and was disturbed by "the sooty appearance of the Moor ... I could not separate the African color from the man, nor prevent that disgust and horror which filled my mind every time I saw him touch the gentle Desdemona".

Guess who said this:

I am not ... in favour of bringing about in any way the social and political equality of the white and black races ... of making voters or jurors of negroes, nor of qualifying them to hold office, nor to intermarry with white people ... there is a physical difference between the white and black races which I believe will forever forbid the two races living together on terms of social and political equality.

I'm afraid that was Abraham Lincoln and no, although he was responsible for the Emancipation Proclamation that ended slavery in America, he never changed his opinion that, "I as much as any other man am in favour of having the superior position assigned to the white race".

But wait, that's just rank 19th Century racism, right? Well, yes, it is from our perspective, looking back with horror at the indefensible views on race held by such great men as Thomas Jefferson, author of the doctrine that "all men are created equal". He also owned hundreds of slaves, and wrote such appalling things about black people as that "comparing them by their faculties of memory, reason, and imagination, it appears to me, that in memory they are equal to the whites; in reason much inferior ... and that in imagination they are dull, tasteless, and anomalous".

The idea that black people are by nature inferior was the moral foundation on which both the institution of slavery and the outlawing of racial intermarriage were based. Whether the justification was explicitly religious, as the apartheid regime in South Africa made it, or founded in so-called natural law, which is where the Nazis placed their racial theories, belief in racial inequality has always had an ethical core. Warped ethics, we now agree, but ethics nevertheless. Aristotle, who pretty much invented ethics, also said this: "Some men are by nature free, and others slaves, and that for these latter, slavery is both expedient and right."

But let's move away from race. In 1894, the South Australian Parliament was considering giving women the vote. A petition from a group of women opposed to the reform argued that "the duties and life of men and women are divinely ordered to be different both in the state and in the home".

Australian women got the vote early (second in the world, after New Zealand, although Indigenous Australians had to wait until 1962). In the UK Parliament, it was still being debated in 1912, the arguments in opposition exemplified by this from Viscount Helmsley:

It is one of the fundamental truths on which all civilisations have been built up, that it is men who have made and controlled the state, and I cannot help thinking that any country which departs from that principle must be undertaking an experiment which in the end will prove to be exceedingly dangerous.

But wait, that's just rank 20th Century sexism, right? Why yes, so it is. Same thing though; sexism comes from the conviction that the male sex is superior. The conviction is ethical, whether we consider it sensible or contemptible.

Which brings us to today and the arguments against same-sex marriage. There's the guy in Lakemba during an ABC TV vox pop repeating the clichéd but crystal clear aphorism that "God created Adam and Eve, not Adam and Steve". Or the Government Senate Leader Eric Abetz explaining his rationale:

I firmly believe that the institution of marriage is one that has been uniquely, over the centuries, the union of a man and a woman to the exclusion of others for life.

These arguments are based in scripture and tradition - the law of God and the law of Nature. They are, accordingly, ethical.

What does this tell us? Intellectually, it reveals that ethics provide unsound ground for any argument that propounds a universal truth. A principle of nature that is, in one age, held to be unarguable, will inevitably at another time be generally considered laughable. Slavery lasted for thousands of years before anyone thought it abhorrent; racial inequality remains a widely held belief, even though it's well established that the concept of "race" is nonsense. Women and men are, likewise, still treated as unequal to greater or lesser degrees in every part of the world. As for the LGBTQIA population, well, there's a long way to go when most religions are still teaching that they'll be damned.

The fight for equality is never done. It moves forward in small advances and great lunges; sometimes it retreats. It is always opposed, and the opposition is always founded in sincerely held beliefs. So it is with marriage equality.

I do understand that there is rank, hate-filled, fear-driven prejudice at play too, in this as in all battles against inequality. However, I also think that most of those who stand opposed, do so believing they are right. Ignoring that only prolongs the fight.

Never mind; deep down, we all know how this ends. One day, my gay friends will be married under Australian law and we'll have the mother of all parties to celebrate. That may be later this year or in 10 years. But it will be.

Michael Bradley is the managing partner of Marque Lawyers, a Sydney law firm, and writes a weekly column for The Drum. He tweets at @marquelawyers.

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