Except that it hasn't at all.<quoted text>
Natural law and history have, for millennia, demonstrated that marriage is a relationship between one man and one woman.
It is YOU who does not agree with this self-evident standard.
Marriage has meant a wide number of arrangements throughout the eras, only quite relatively recently coming to mean a man and woman choosing to partner with each other.
Historically, gay and lesbian marriages have existed in a wide number of cultural contexts. As have concubines, multiple marriages, warbrides, really a dizzying array of interesting options!
The idea that marriages have mirrored a modern structure throughout the ages is one born of poor historical knowledge and a failure to do basic research.