Marriage IS a legal issue.
The Catholic Church won't marry divorced people whose former spouses are still alive, because they still consider the couple married -- but the same couple can be legally married, because the state recognizes the divorce.
On the other side of the coin, some religions and churches recognize gay marriages in states where it isn't recognized, so people married in these religions/churches are not recognized as married, even though the congregants do.
If a business man doesn't want to make a homosexual wedding cake, so be it, it's his prerogative to refuse service to anyone he/she wishes. Ask any bartender.
Ask if the bartender can refuse to sell to a gay patron, in the same state where a bakery has to sell cakes to a gay customer.
The point being, there's this whole separation of church and state, that means the state is to mind it's own business and leave the religious issues to the church.
Marriage isn't exclusively a religious issue, and hasn't been for a long time.
How would you suggest the state get out of the marriage business? What happens to the status of currently married couples? What happens in the future -- do church marriages have any legal recognition? Can any group conduct marriages? Is there a legal contract analogous to marriage that the state recognizes? If so, what happens if people still refer to that new legal status as "marriage"?