Not at all. The effects of marijuana use pose no serious threat to the user or to society. Therefore, marijuana use should not be criminalized. If anything, I'd rather the federal government focus on more pernicious drugs, like cocaine, heroin, and meth.
You're saying they DO pose a threat to the user, just not a serious one? I agree, and that's one reason to prohibit it.
As to the "threat to society" -- IF the legalization of marijuana resulted in a doubling of teen use, would that be enough of a threat to our society to maintain the illegality of marijuana? Or don't you care about that?
IF the legalization of marijuana resulted in a tripling of marijuana use among adults (with no reduction in the use of other drugs, like alcohol), would that be enough of a threat to our society to maintain the illegality of marijuana?
I'm just trying to get a feel for your definition of a "serious threat to our society", which I'm guessing is a very high bar (I thinking your "serious threat" = Bubonic Plague).