Here and now, the liberals are tearing our country apart. Here and now, it is predominately liberals who favor the legalization of marijuana. Therefore, here and now, I oppose the liberals in their efforts to further degrade America with weed, as I oppose them on every other issue they push.
You posit it as a conservative/liberal issue, but what I'm suggesting is it's really a statist/non-statist issue instead.
You're concerned with the societal effects of marijuana use, and that's a legitimate concern. However, this makes you no different, in effect, from a liberal who is concerned with the societal effects of poverty, or a liberal who is concerned with the societal effects of income disparity between blacks and whites, or a liberal who is concerned with the societal effects of not having access to free contraception through one's law school health care policy. In all cases, you're clamoring for a heavy-handed, proactive government solution to a societal problem that is best handled in an EXTRA-governmental manner.
You want to warn people of the societal effects of marijuana use? Have at it. Educate people: Broadcast your message far and wide. Use facts. Argue your case. Make yourself heard.
Don't ask the government to do your dirty work for you, lest you give liberals and progressives the standing to get the government to do their dirty work for them as well.
Get it?