Government at all levels should get out of the business of defining marriage. The only involvement of the state in marriages or civil unions involves contract enforcement; this has nothing to do with the sanctity of marriage. There is no reason why this should be any different for same-sex couples.
The laws, where necessary, should be changed accordingly, and marriage licenses should use some neutral term. When the license is issued, the contract should be in force without requiring further action by a church or civil authority. If the couple chooses to have a ceremony, they can find someone who will do it.
Same-sex couples have the right to call their relationships marriages if they choose to, but they don't have the right to force everyone else to do likewise. Others have freedom of speech too.