"Marriage" is a religious sacrament & should therefore not be controlled by any govt org.The US Constitution clearly provides for separation of church & state.
The govt should define & control only the legal contract between 2 people which should be called "Civil Unions" (CU). A legally defined CU should be available to any 2 people regardless of race, color, religion, gender (same or diff), national origin, disability, who are are not blood relatives (1st cousins or nearer) & of legal age.
CUs should be defined at the Fed Govt level. All State & Local govts should be required to issue licenses per the Fed Govts definition. CUs should provide the same legal rights & responsibilities as currently defined for Marriage. Termination of a CU should be a Divorce or the equivalent.