Marriage was originally a religious concept and should remain so.

The government got involved because as populations grew and changed the old way of tracking births, deaths and marriages at the local parish became tedious and inaccurate -- government took over the job so that it could be centralized (and to take over the revenue). Government's legitimate role in "marriage" has solely to do with property rights and child custody disputes. It is solely an enforcer of the contract.

Therefore, government should get out of the "marriage" business all together. Unions, between whatever combination of individuals, should be recorded with the government as a contract -- essentially a required prenup. The government should care what gender those individuals are or how many of them there are -- it should simply record the contract and enforce it if/when the relationship ends.

Marriage is between those individuals and their church -- if their church recognizes the union as a "marriage" under it's rules, then it is so.