I'm no grammar nerd but I think the comma makes "sex" an adjective, which should make me able to say that the drug-infested dens are "sex". this says a comma only splits coordinate adjectives, ~~which can be swapped and~~ where the latter does not form a common union with the noun, which means that even if "sex" were an adjective, the comma implies it can be swapped to make "sex, drug-infested dens" (single noun phrase), which it can't because sex is not an adjective.

Edit: this proof by contradiction doesn't quite work because adjectives have a preferred order by category of what they mean, and being coordinate doesn't mean they are in the same category.