But marriage is a concept. There are no actual physical characteristics of marriage itself. People enter into marriage, they are not actually part of what marriage is.
Marriage does not have to be between a man and a woman only. That is a restriction on legal marriage, not marriage itself. I can prove that with the definition of marriage.
a (1) : the state of being united to a person of the opposite sex as husband or wife in a consensual and contractual relationship recognized by law (2) : the state of being united to a person of the same sex in a relationship like that of a traditional marriage
From the Merriam-Webster dictionary. Marriage - Definition and More from the Free Merriam-Webster Dictionary
The legal restriction of marriage being "a man and woman only" will change because it is not involved in what marriage actually is, so therefore maintaining that restriction does not further any legitimate state interest.