For me it’s driving while under the influence. If you couldn’t tell, I like me some ganja. However I have long since held the belief that it is utterly insane to drive while under the influence of most substances, with maybe nicotine and caffeine being the exception. All too often I see other stoners smoking and driving, which I simply can’t fathom. I’ve only operated a vehicle once under the influence and it was just to move a U-Haul around the block to a different parking spot, which was such a scary experience while high that I refuse to even consider getting behind the wheel again while high.
DnD is definitely one of those hobbies where each table is different and (as long as no one is being hurt) none of them are wrong. The toxicity some people bring to the table can scare away newer people entirely and that sucks for everyone. Less DnD people means we all get to play less DnD.