Real marriage has always been and will always be a committed relationship between one man and one woman. Pretending or saying it ain't so doesn't make it so. Anything else will just join the list of fake marriages.
It is the only relationship that reproduces naturally (including a father and mother raising their children).
It is the only relationship that is the birthing place of every single other type of relationship.
It is the only relationship that reunites two completely unique parts.
It is the only relationship that sexually fit together by design.
It is the only relationship that restores a male and female to the original roots of our creation (pre-gender).
It is the only union that blends two different genders bringing perfect balance. A same gender union would lack diversity and be off balance.
All this says nothing about the culturally historic and religious distinctions.
It clearly has, needs and deserves a special and unique definition.