Many heterosexuals oppose marriage but favor domestic partnerships. Separate but equal is not equal. The rights, responsibilities and benefits of marriage cannot easily be recreated. Many people have a confusion because, in America, marriage is two things. There is religious marriage and civil marriage.
Civil marriage is the contract with the state that creates a family.
Just as some religions do not recognize civil divorce, when the state eventually recognizes civil marriage for same-sex couples, no religion would be forced to do the same.