The distinction is in that the majority of parents that drink, do so responsibly, and with no ill affect to their family; handling money irresponsibly is simply a trait, not an effect of using a substance; multiple sex partners.... (I have been known to consider laws against adultery for married couples with children. Even Uriel has mentioned it. It is, after all in the civil commandments of the lord. however) engaging in such activities does not prevent you from acting responsible around your kids, family, or neighbors.
I think we're nearer to the heart of the issue now. You say that the majority of alcohol users are responsible. The assumption is that, conversely, the majority of drug users are irresponsible. However, is this borne out by the facts? I would argue that to determine whether this is true you would have to compare the total population of drug users (which I think we can all agree is larger than we know) to the percentage that commits other crimes (robbery, domestic violence, etc.). If the comparison yields numbers equal to or higher than that found in alcohol users, the argument that drugs are simply too dangerous to permit may have some merit.
I don't have the stats at my fingertips, and I don't even know that accurate data exists to make this comparison. My point is simply that our information about drug users, especially in the mainstream news media, focuses on those users who do commit other crimes and otherwise make an utter mess of their lives. Is this because they truly represent the majority of drug users, or is it because they're the ones getting caught on account of their stupid, self-destructive behavior?