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.
The notion that marriage has always been the same for hundreds of years is a complete furphy.
And the one constant through all that which you described is that "marriage" has been between a man and a woman.