Maybe a euphemism, but not about prostitutes per se. The term “slack posture” is merely a metaphor for loose morals in general. It’s purely literary, in the same way that a person could be described as “standing tall” in some moral way but not actually be physically so tall. Could it have been that the term “slack posture” was code for something else–some indicator which people in that particular society would attach to, let’s say, low-income status? (C.f., slack-jawed yokel.)