And the one constant through all that which you described is that "marriage" has been between a man and a woman.
No, at times it has been defined as being between one man and several women (polygamy), and as well since 2002 marriage has been described as being between two people of the same gender. Let's not forget the English speaking nations (sans Australia), Western Europe and even places like Brazil, Mexico and South Africa have legal marriage equality. It isn't a concept that is never existed before. A good chunk of the developed world has it, and the sky hasn't fallen in.