At the risk of being indelicate, one could well say that a law on the books changing the definition of marriage will be as successful in making the “union” between two men, or two women, into a marriage as a law declaring a gay person’s genitals to be the genitals of the opposite sex would be.
Is marriage only a person’s genitals? No. But it’s comprised of a man and a woman, and I’m using the biological point to illustrate something that, until very recently, was perfectly understood by everyone.