"Stick with nature"
Probably one of my least favourite arguments for legalizing/anything ever.
It doesn't make sense, there is nothing inherently good or healthy about natural things. Poison frogs live in nature, and you don't lick them. You will die. Some good and bad things exist in nature and in synthetics. People who think that natural things are automatically better than unnatural things don't realize the way evolution works.
Also the whole argument about what is natural/unnatural... by this logic honey would be unnatural wouldn't it? Either that or human shit is unnatural. Choose your own adventure!