The term "marriage" should no longer be a word that appears in state law.
A term like "civil union" should be used, and it would essentially be an exclusive legal contractual obligation between two consenting adults (and only those two adults).
This could be a man and a woman, a man and a man, a woman and a woman, two brothers, a brother and a sister, a mother and an adult son, etc...
There would be no implication of love. There would be no implication of sex. The state is NOT and should NOT be in the business of issuing licenses for LOVE or SEX.
This union would give the two individuals certain tax rights, property ownership rights, inheritance rights, and decision rights, similar to what is currently seen in statutory marriage. This union would be a legal contract, just as two businesses join through a legal contract, or just as a salaried employee enters into an employment contract with their employer.
The state would NOT recognize religious marriages, unless the party marrying in the church filled out the proper paperwork for civil union and it was filed in the city hall or whereever.
That's the way it should be.