In reflection of your post and the comments you made here, OP, it sounds to me like the fact that your son is a straight-edge, as well as his exposure to bad behavior/drugs/criminal acts, has lead him to acquire an unrealistic view of the people who he would consider "good". If you have been one of the sole lights of his life, for him to find out that you engaged in ANY behavior similar to the behavior of the people that he has grown up to consider "bad" has probably come as a total shock to him and he is disgusted. He probably thought you never did anything like that in your life. I think what makes it 10x worse is the fact that you didn't say you did it when you were a teen, you said you did it in your 40s! He's probably thinking: "She knows how hard our life has been, how much pain drugs have caused, how fucking awful this whole mess is, WHY WOULD SHE EVER SMOKE POT. WHAT WOULD SHE DO THIS TO ME!?!" He is now lashing out at you as a result of his unrealistic view of you being shattered.
I do agree with another comment, however, that if he found this out at the age of 25 or something, that his reaction would be totally immature. I think because he is only 17, he is immaturely clinging to what he thinks makes sense and unfortunately, you are now feeling it.
Perhaps instead of punishing him, you could sit down with him in a serious and objective manner, and explain to him that you think you understand why he is upset, and would like to him to explain it himself. It what I said above is true, you could try to explain to him that although you understand it must be painful for him to learn that you went near ANYthing in recent years that had to do with doing drugs, that what you did and what the actual abusers of your family did/are currently doing, are two entirely different things.
What he is doing would be like if my kids (if I had any) were angry at me for having a glass of wine because my dad was an alcoholic. To someone who doesn't completely understand that recreational use is totally different than substance abuse, it can be inevitable for them to consider the two as the same until/unless it is explained and to them otherwise. Like most teenagers, this is something he isn't going to understand from just talking to him; he will have to come to the realization on his own, but for now, presenting your thoughts as well as the correct facts in this matter in an ADULT, calm, objective, manner, might be just what he needs to start seeing the light. Therapy sounds like a great idea, too, because he is obviously being affected by the negative influences he is forced to be related to in a much greater way than you might see. Trust me, as a person who grew up around addiction, it has 100% impacted the kind of person I have become, for better or worse.