Now I think we've wandered from what is a clear case of sin, for one can point to clear, concise texts that condemn fornication as sin, to areas that are more matters of Christian liberty and Christian conscience.
Our Lord told us that it's not what goes into our mouths that causes sin. I don't think there is any substance that is sinful in of itself. Many things can be for both good and bad. Sugar can make a cake that tastes sweet on a person's wedding day, it is also a source of energy for the body. It can also kill a diabetic. Morphine can give a sick person rest and a respite from pain, or cause someone who should be hale and hearty to be a wasted addict. It's not the substance itself, it is what we, as partakers of the substance does with it.
The most important thing about pot is that it is illegal. So, because we as Christians are to obey the laws of the land, it is a violation of God's commandment to smoke it. Should Christians work towards over turning marijuana laws? I won't, but it's a matter of one's conscience. I can't judge another's heart in the matter.
The other aspect of pot is, while there are those who can drink a glass of wine with some spaghetti, or have a beer with some bratwurst sandwiches, and drunkenness is no part of it, there is usually just one reason and one reason alone to smoke pot and that is to get the buzz. And, as we are commanded not to drink to the point of drunkenness, we can easily apply the spirit of the law and not smoke to the point of being high. But, if one isn't smoking pot to get high, just what is the point? Who smokes pot because it tastes good?
However, my thread is more about concrete sinful activity. There are no "two sides" to the story of fornication. It is clearly sinful behavior, and one that one Christian parent should be able to expect other Christians parents to not allow when one's child is at their house!
In response to the current threads that are making the assertion that there is nothing wrong with getting stoned, then I would say that there should be nothing wrong with getting drunk as well...and yet drunkenness is clearly condemned in the Scriptures. I can dig up the chapter and verse if you need me to.