Right now many people wrongly think the term 'marriage', as used by state governments, implies approval by a religious authority. This is not true, so government institutions should not be allowed to use the term in a legal context.
In America, the State does not have authority to sanctify anything, including marriage; that's the domain of religious institutions. So leave 'marriage' (the sanctification of a relation between two people) to churches. Federal and state governments should only be able to use the term 'civil union' (a contract binding two individuals' property, hospital visitations, and tax obligations).
This proposal is inspired by chapter 15 of Thaler and Sunstein's book Nudge, which argues the case in detail.