The same-sex marriage debate between senators Cory Bernardi and Penny Wong at the National Press Club exposed four myths about marriage that have plagued recent debates worldwide.

The Press Club debate was framed in terms of social justice versus tradition. Wong argued that current marriage law is unjust because it inscribes discrimination based on gender alone. This would be unthinkable for any other personal attribute such as race or ethnicity.

Wong claimed that legalising same-sex marriage will not change the institution of marriage. It would, however, be a clear statement that lesbians and gays belong, are accepted, and that their relationships “matter too”. It would be a sign that:

Australia has become an accepting, diverse and inclusive nation.

Bernardi argued that marriage has always been between a man and a woman. It is a sacred bond and the foundation of the family – the “basic unit of society”. He asserted that same-sex marriage was unnecessary, as all discrimination against same-sex couples was removed in 2008.

Framing same-sex marriage as a “textbook propaganda campaign”, Bernardi suggested that it was based on self-interested desires rather than social justice. A child’s right to a mother and a father outweighed the desires of same-sex couples to have their relationships recognised as marriages by the state.

And, reprising his notorious slippery slope argument, Bernardi suggested that redefining marriage would lead to the incremental redefinition of marriage to include multiple partners, and to attacks on religious freedom.

Wong’s argument was more warmly received by the audience. But both she and Bernardi deployed deeply seated myths about marriage that don’t stand up to historical analysis.

Myth one: marriage is a timeless, changeless institution

For almost a century, it has been widely accepted in histories of love, marriage and the family that, even in the Christian West, marriage has changed dramatically over time.

Christian literary critic C.S. Lewis famously argued that romantic love was “discovered or invented” some time in the 11th century. As Wong correctly claimed, marriages were historically organised around economic and political priorities. It was only from the late 18th century that people began to marry for love.

As my recent book shows, expectations of marriages based on mutual affection and romance did not become commonplace until the middle of the 20th century.

The meaning, purposes and expectations of marriage in the West have changed dramatically – even in recent times.

Myth two: heterosexual monogamous marriage is normal

Bernardi argued that marriage has always been a relationship between a man and a woman for the purposes of reproduction. However, in the global history of marriage, heterosexual monogamy is not the norm.

Outside of ancient Roman society and Christendom, almost all societies have historically practised some form of polygamy or concubinage. Pre-Christian Western Europe was polygamous before the introduction of Christianity. And even modern Christian leaders have supported polygamy.

Martin Luther, a hero of the Protestant Reformation, said that he could not:

… forbid a person to marry several wives, for it does not contradict Scripture.

In 1988, the Lambeth Conference of Anglican bishops recommended that those in polygamous marriages could be baptised and received into the church.

Others, most notably historian John Boswell, have argued that there is a history of ritualised same-sex unions that stretches back to premodern Europe.

Historically, and even within the Western Christian tradition, heterosexual monogamy is not the timeless tradition that conservatives think it is.

Myth three: marriage is a religious institution

Both Bernardi and Wong emphasised the sacred nature of marriage. However, marriage is not, even in Christian theology, made by the church.

Marriage’s essence is the promise that two people make to each other in entering a relationship. This is the case even in “Christian marriage”. As historian John Gillis demonstrated:

Church and state ceremonies are relatively recent additions, which have been grafted onto older popular rites whose legitimacy was dependent on no written law.

Over time, churches took on the administration of marriages. But this was for a relatively short period. In the UK, for example, the church had sole jurisdiction over marriage only between 1753 and 1837. Marriage’s secular essence was even acknowledged by the Anglican primate of Australia, Archbishop Philip Freier, in a recent opinion piece.

While churches, mosques or synagogues might bless nuptials, marriage itself is not a religious institution.

Myth four: same-sex marriage won’t change marriage

Wong claimed, to considerable laughter, that the passage of same-sex marriage wouldn’t affect heterosexual marriages. However, marriage is not a timeless, fixed institution.

In her opening statement, Wong suggested that the campaign for same-sex marriage was the product of earlier social movements for equality in marriage. In support, she cited the historic overturning of bans on interracial marriage and shifts towards mutuality and equality in heterosexual marriage.

If historical changes towards racial and gender equality in marriage made same-sex marriage thinkable, we should not be surprised if the greater equality embodied in same-sex marriage makes inequality in opposite-sex marriage less thinkable.

This is at the heart of conservative opposition to same-sex marriage. Bernardi claimed that children have a right to a mother and a father, and that the complementary differences between men and women are necessary for ideal child-rearing.

I would flip this argument on its head. For conservatives like Bernardi, “traditional marriage” is necessary to maintain differences between men and women. The redefinition of marriage to include same-sex relationships is not, as he admitted, a threat to children. But it is a threat to a gender order that privileges men.

Perhaps, for some, there is something to fear from equality.