I'm shocked that you arent a fan of sin tax.
But fat people cost the government more than thin people in terms of health care costs, and fat people also consume more soda. The link between sugar and diabetes is pretty fuging clear, maybe this societal cost should be including in the cost of your sugar water?
Well, that is what being libertarian really is. We should be beholden to our own life's choices and to each other. Government is not supposed to be a parent to tell us right from wrong. They should protect our rights and very little beyond that. Once you are in the business of legislating morality, you are stepping into all the traps and pitfalls that have plagued civilization since our beginnings.
It doesn't mean that community doesn't have their own purview. It simply means that government's responsibility is to ensure that community does not infringe upon the rights of the individual/minority. Otherwise, majoritarian tyranny reigns as it almost always seems to do with pure democracies. If I wish to do something that is bad for me and no one else, that is my right. The converse of that is that I alone am responsible for the consequences of my actions.
i.e. - If I am irresponsible for my health, it is not the responsibility of everyone else to pick up the tab for my carelessness.