The biggest argument for religious people against same-sex marriage is that it opposes the Biblical definition of marriage. They believe a marriage is between a man and a woman.
The biggest argument for same-sex marriage advocates is that granting them civil unions versus marriage creates a "separate but equal" status, which is not really equality.
If we changed the title of a "marriage" granted by the government to "civil union" for all citizens, regardless of sexual identity, it would satisfy both the arguments listed above. Then it would be up to each individual religious organization whether they wanted to grant the couple a "marriage" or not, but both sides would be equal in the eyes of the law.