Many people argue that the government should regulate/prohibit only those activities that harm other people, not activities that harm solely the people who freely and voluntarily consent to engage in those activities.
For example, many people would say the government should prohibit murder, but not smoking weed.
Part of this is based on the idea that it is really difficult and paternalistic to say that an activity "harms" the person if they voluntarily chose to engage in the activity.
But there are many activities that have no tangible, harmful effect until many months or years down the line, at which point the effect becomes quite serious. If the government has evidence that 10 years down the line, 70% of people who engage in a specific activity regret their decision and wish they hadn't engaged in the activity, does that give the government grounds to prohibit or regulate the activity?