Why do some people who would recognize gay civil unions oppose gay marriage? Certain religious groups want to deny gays the sacredeness of what they take to be a sacrament. But marriage is no sacrament.
Some of my fellow Catholics even think that “true marriage” was instituted by Christ. It wasn’t. Marriage is prescribed in Eden by YHWH (Yahweh) at Genesis 2.24: man and wife shall “become one flesh.” When Jesus is asked about marriage, he simply quotes that passage from Genesis (Mark 10.8). He nowhere claims to be laying a new foundation for a “Christian marriage” to replace the Yahwist institution.
Some try to make the wedding at Cana (John 1.1-11) somehow sacramental because Jesus worked his first miracle there. But that was clearly a Jewish wedding, like any other Jesus might have attended, and the miracle, by its superabundance of wine, is meant to show the disciples that the Messianic time has come. The great Johannine scholar Father Raymond Brown emphasizes this, and concludes of the passage: “Neither the external nor the internal evidence for a symbolic reference to matrimony is strong. The wedding is only the backdrop and occasion for the story, and the joining of the man and woman does not have any direct role in the narrative.”