The idea that marriage is granted by the government doesn't really sit well with me though. It should be a religious sanctum
Legal marriage exists. Technically, getting rid of marriage as a legal term could be constitutional (as long as legal unions for gay and straight couples are equal), but it's not realistic. Straight married couples won't be pleased if they're suddenly no longer married and just have a "civil union".
It seems that opponents of SSM are just proposing this now (even if you support SSM) only because they lost the fight to impose their religious beliefs legally. I don't think they like the idea of liberal religious institutions being allowed to perform gay marriages.
