Okay first off. Junk food isn't illegal, nor is drinking after the age of 21. Both of these are not valid for your defense seeing you were trying to show illegal activities that are bad for you.

Second, I can change lanes all day w/o using my blinker and as long as I don't hit another car or another car hits me, I'll be okay, and never damage my health, terrible comparison.

Third, a better argument would have been smoking cigars, cigarettes, or any form of pipe. These are LEGAL and bad for you, and despite the fact they are more harmful, they are still legal. But you didn't use that one, so to late.

But what you are saying in your argument instead is that we do plenty of shit that is bad for our health, so why is he using health issue against weed. So basically, because someone does something unhealthy, why not do one more thing thats bad for us, because, whats one more unhealthy thing right?