Marriage predates Christianity.
Of course it does. Everybody knows that. Jesus Himself said it was from "before Moses," that it was "from the beginning."
What Jesus did was to restore and ennoble it even further: One man. One woman. For God. For good.
“Marriage predates Christianity...”
True. Marriage goes back to the creation, Adam and Eve and was instituted by God the Father himself, before the 2nd person of the Trinity (Jesus) was incarnated.
Marriage is as old as mankind and has nothing to do with governments, Ceasar, etc. It is a sacrament that the man and the woman bestow on each other. Governmental interference throughout history is inevitable (because we live in a fallen world) and perhaps even necessary (banning polygamy, for example), but marriage is a sacred covenant between the man and the woman.
Nothing predates the one who created all things that have been created.