One of the skeletons in the Progressive Education closet is Scientific Racism, otherwise known as Eugenics, which the leaders of the Progressive movement enthusiastically espoused until the Nazis in Germany gave it a bad name.
It's not that the Left is "redefining" marriage — it's that conservatives must redefine the debate. The question in this debate should never be one of rights. It should be one of definitions. If we accept that marriage is, by definition, the union between a man and woman and nothing else, the faux-marriage-rights argument is moot.