Marriage and the State

With what right does the state say who can and cannot marry? The state has, at various times, said that people of different races cannot marry, that people of the same sex cannot marry, that no one can marry more than one person at at time. But with what legitimate authority can the state make such prohibitions? John and Ken welcome Richard Mohr from the University of illinois at Urbana-Champaign, author of The Long Arc of Justice: Lesbian and Gay Marriage, Equality, and Rights.