Historically from about 1250–1300 CE "marriage" was a union between a man and a woman. That was long before the Government clarified the definition in the Marriage Act to reflect society's long standing view of a "marriage."
In the opinion of a lot of Australians, that definition still stands.
Marriage since 1300 has also been defined as being a relationship between two people of the same race only, or of the same social class or faith, or who's partners were chosen by their parents, or where unions between cousins were very common, or where multiple partners inside or outside of marriage were common and accepted, or where marriages were almost solely performed in churches only, or where women have had lesser legal rights, or where having children was mandatory, or where no fault divorce didn't exist, or where a woman couldn't vote or legally own property, or where a man could legally assault his partner.
Some of those definitions have only changed very recently (Queensland only criminalised assault in a marriage in 1989!).
The notion that marriage has always been the same for hundreds of years is a complete furphy. It has evolved and changed as society has evolved and changed and will continue to do so.