It shouldn't be any business of the 'state' to be able to dictate our moral and cultural values, whichever side of the fence you sit on. It can only have laws to prevent us from actually harming the livelihood of others. That's why SSM is not as clear cut as some may think.
It's job is to ensure that everyone in society has equal rights, particularly legal rights. Currently they don't.
From that perspective SSM is about as clear cut as can be.