Personally, I have a philosophical problem with prohibiting so-called gateway behaviors - i.e. a behavior that's innocuous in itself but that could potentially lead to a bad behavior. For example, nothing wrong with having an occasional drink (at least IMHO). Of course everyone who's a drunk driver is by definition someone who drinks and having one drink could certainly lead to having 10 drinks and then getting behind the wheel of your car. Does this mean that you make alcohol illegal? No, you make drunk driving illegal.
Similarly, the legal ownership of guns creates the possibility that someone will use that gun illegally - i.e. shoot another person without justification. Does that mean you make guns illegal? No, the ownership of guns in and of itself doesn't hurt anyone. Instead you make it illegal for a person to shoot someone else without justification.
Seems to me the same is true of smoking marijuana, looking at pictures of naked women on the computer, playing violent video games and a host of other activities that some people want to (or do) prohibit. You shouldn't prohibit an activity that doesn't harm anyone (except possibly yourself) simply because 1/10 of one percent of the people who do it go on to engage in activity that is harmful to others.