Incentivizing bringing either physical, emotional, or social harm to yourself.
For instance: we don't allow people to work for less than minimum wage. We have things like OSHA which set minimum safe working requirements for dangerous jobs, etc.
You may claim "yes, that's why we want sex-work regulated".
Sex work already is regulated. Pornography is legal, stripping is legal, "cam" work is legal, burlesque is legal, etc. etc.
The classic example is: suppose a drug addict who has a moral opposition to selling their body for sex, but is being coerced into doing so by their drug addiction. We have protective regulations around that which strongly disincentive it.
Selling penetrative sex for money is where we as a society have drawn a line and said "everything up to this point is okay."
Just like we drawn a line for dangerous working conditions and said "everything up to this point is okay". We're not saying that there are no risks to dangerous physical work (sex work included), we're saying that anything beyond the line is intolerable.
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Just to be clear: I'm not making some claim that strongly supports any side of this argument. I'm just annoyed by the claim that there is no discussion to be had here, and that the only reason that prostitution is illegal in most of the US is "Christians".