I consider myself relatively sane, and I can say with certainty that if heroin or cocaine were legalized today, I would not be rushing to the store to get some. I don't know any responsible adults who would either, just because it would be legal. Do you fear that you would be unable to control your own impulses and start shooting up?
No, but people and kids that grow up with these drugs sitting on local store shelves will eventually see nothing wrong with using them. I know many people that are now drug free because they no longer like the danger of being arrested.