Let me explain why pre-marital relations, pornography, drugs and alcohol SHOULD remain illegal. All these vices are done for the "on-demand", "any-time" "high" and "pleasure" they produce in the brain.
The brain is all about reward and consequence, pain and pleasure. That's how the brain works. The brain is designed to reward a person with pleasurable and loving sensations for safe, smart and productive behavior. And on the other hand, the brain is made to punish damaging, unsafe and destructive behavior by producing pain and fear. When we feels these sensations, our axons form new connections that reinforce certain memories, thought processes, emotional responses, and behaviors. However, when a person turns to drugs or other vices and "psychotropics", that person is cheating the brain. Psychotropics bypass the normal reward centers of the brain to feel "high" unnaturally. Or in other words, people who use drugs are feeling "high" without earning it.
Feeling good should be earned, not stolen. Pleasure should be the reward for hard work, achieving goals, reinforcing marital relationships, learning something new, or helping your neighbor. The laws of the land should never promote the practice of people "unnaturally" giving themselves "on-demand highs" and "instant gratification" as a reward for sitting on the couch and smoking something.
law should reflect our ideals and not the least common denominator. That said, just because something is illegal doesn't mean we punish people like we are doing now. Joseph Smith wanted to reform our Jails. As far as the argument about those with a "higher understanding" enforcing their morality on those of "lower understanding". One requirement for law is that all people under that law must be educated to understand the law. By being clear about the purposes behind the law, the law itself can educate everyone about the higher reasons for the law.
Remember that fornication and drugs and such have been traditionally illegal. So, I am taking the conservative and traditional position. However, obviously, we have forgotten the basic reasons why drug use is wrong and damaging and unnatural and it is failure of the good people of this country to accurately explain the wisdom of the law, that has opened the door wide for debate.
Our laws, be they from a king, or by representative democracy, should reflect the laws of God. It is this point alone which make our laws good or bad in the eyes of God. Having a republic with Bad laws and bad law makers isn't any better than having a corrupt and evil king if the laws are just as corrupt and evil. All laws involve the enforcement of morality. The question is, whose morality do we enforce? The Book of Mormon teaches that we are to judge men according to God's law and according to God's understanding. I don't think it can get any higher than that.
Mosiah 29:13 Therefore, if it were possible that you could have just men to be your kings, who would establish the laws of God, and judge this people according to his commandments, yea, if ye could have men for your kings who would do even as my father Benjamin did for this people—I say unto you, if this could always be the case then it would be expedient that ye should always have kings to rule over you.