No more dangerous than riding a horse = not unlikely to put a novice in the hospital
Don't get me wrong. I think modern society has way too little tolerance for risk of physical harm but "less dangerous than horse riding" isn't all that safe as far as indoor activities go.
The comparison is meant to tug at people's intuitions: people let their daughters ride horses. So unless we're about to outlaw horse riding, we ought to not outlaw ecstasy.
Don't get me wrong. I think modern society has way too little tolerance for risk of physical harm but "less dangerous than horse riding" isn't all that safe as far as indoor activities go.