Earlier quoted context omitted.
It's said to objectify women. More specifically, the Johns are treating women's bodies as something that can be traded commercially. The more that is normalised, the more it damages all women - not just prostitutes. If you don't think objectifying women harms them, then that's not a "good" argument, I guess. FTR, I'm against laws controlling consensual sex. Not all harmful behaviour should be criminalised.
Bodies are used commercially all the time, such as in manual labor, physical therapy, disability care, security, policing, military service, athletics, dancing, commercial modeling, ... The problem that's particular to sex work is legal systems that fail to prevent coercion. This is exacerbated by religious advocacy groups that try to make sex work harder without making it safer.
It seems that you equate sex to manual labour, when it is one of our important biosocial functions (if not the central) that involves much more complex reactions and distortions at all sides (a prostitute, a client, an aware neighborhood/society) than kicking a shovel into the dirt or massaging a muscle. It’s always baffling how some people try to render it as just a mechanical process akin to workout and/or blowing a nose. If it’s kind of the same, why hiding it from kids and not serving clients right on the squares, like hotdog stands do.
gp: The more that is normalised, the more it damages all women - not just prostitutes
I’d argue it damages society as a whole. Even if prostitutes and all women could be fine and safe by some magic mean, distorted concepts of a “succesful social woman” hit men back as well.