You ask: From rights of survivorship to hospital visitation, there are matters of basic decency which can be settled in law without redefining marriage at its root.
This is exactly why legal marriage is necessary, because basic decency has been violated continuously against same sex couples. And letting same-sex couples wed does not redefine marraige at its root. Marriage remains a personal contract between two unrelated persons which establishes kinship. The state is interested in this because marriage stabilizes society, and provides security for the members of the family - the couple, any childen they bear, adopt or bring in from a prior marriage.