So you're OK with equating marijuana with opium, even though they're not even chemically or pharmacologically related.
Neither is Sarin or Anthrax, but they both lead to the same result. Dead people.
Marijuana is the most minor form of available narcotic drugs and it often kickstarts subsequent addiction to worse and more dangerous substances.
Tolerating Marijuana is a lot like tolerating socialists. If you put up with them, Eventually they will turn into communists.
Apart from that, if you accept the principle that adults have a right to take marijuana, then by what argument can you deny the legalization of all other drugs?
You erode the foundation, and the whole house collapses. Even the tiny little trickle of Marijuana legalization will eventually break down the dike holding the others back.
And then the nation dies.