The idea that sex workers are ‘used’ or that their bodies are commodities is a fallacy. But many feminists use this argument to claim that sex work is degrading, anti feminist, commodifies women or is harmful to them. Moralists (who are sometimes indistinguishable from the radical feminists) use the argument to justify looking down on sex workers or pitying them because they’re “degraded”. The radfem myths of ‘false consciousness’ and sexworkers’ lack of agency are also heavily dependent on seeing them as used bodies, as sex slaves.
But if you think that sex workers are used by clients, that idea is actually made up of several patriarchal ideas about gender and gender rules.