I think @WasCy puts it very well in the first line above: “I don’t need to be protected from myself.”
The problem is when we attempt to enforce moral norms by legislating the behavior. For me, when laws or regulations are passed that attempt to influence or prevent behavior, the question that must be asked is “Does this attempt to prevent an individual from actually or potentially shifting the cost of his behavior to others.”
For instance, I am not against vice taxes like those on cigarettes, although I reserve judgment on whether they work in practice. An individual’s choice to smoke is one that should be allowed, but more in the sense that it should be tolerated. A vice tax is properly placed when the purpose of it is to offset the increased health care costs such individuals impose on society. Certain individual choices such as smoking impose costs not only on the smoker but also on those around him that are often not taken into consideration when the choice is initially made.
The problem with decriminalizing behavior that is harmful like smoking but currently illegal unlike smoking is that there really are no good intrinsic arguments to make arguing for decriminalization in most cases. The fact that prohibition was the law of the land once but now isn’t is, in many ways, unfortunate – imagine growing up in a world without liquor. As much as I love my drinks, I can’t argue that we would most likely be more healthy, emotionally and physically.
Prohibition failed, ironically, not because the policy itself isn’t arguably the best one for us, but because people will inevitably do what feels good. Not all people, and not all the time – but many if not most, and often if not frequently.
“Morals” legislation fails because we do not act morally all the time, and that’s a hard sell, because we are symbolically yielding to the worst in us, and giving legal license for bad behavior.
Of course, there are plenty of arguments about the incredibly harmful unintended consequences of morals legislation – organized crime, gang violence, secrecy in addiction, etc. The problem with bringing these into the discussion is that we expect criminals to profit off of the “degradation and suffering” of others (as those who sell drugs and pimp prostitutes might), but once decriminalized we create a market for both our government and citizens to profit from something that, understandably, most find morally distasteful. All the potential benefits from shining a legal light on the black market (protections for men and women sex workers, profit from taxing drugs being injected into recovery and prevention programs) have to come back to this unseemly fact – we are funding the solution by selling the problem.
The problem in the end is not whether, I believe, most morals legislation should be decriminalized – it’s figuring out how to sell recognition of our own depravity as a benefit.