Marriage Amendment is medieval thinking
Not one of us, for any reason whatsoever, should want to create a society that marginalizes and blames others who are basically us. Race, sex and sexual orientation are all incidental differences that add color to existence. They are not errors in behavior or creation.
"Marriage" is only a word, and words have changed meaning, faded away and even died for centuries.
Furthermore, marriage as an institution is simply a social function. Social institutions work to shape our lives in one way or another, to be helpful.
Social institutions change over time, as is necessary or just. Slavery was a social institution of the Romans, the Vikings and the southern plantations. And, serfdom held three-fourths of the population of medieval Europe under the thumb of the remaining one-fourth who were wealthy. Both seemed absolutely necessary and of unquestioned rightness. It was the way things should be. But the way things should be changes as civilizations become wiser, and when their people are better informed.
Today's marriage amendment is medieval thinking. If there is a current problem that should worry us, it is the dark side effects of religious fundamentalism, rather than an individual's race, sex or orientation.